Katrina Steiner and Hanse Davion win the great game and Victor Ian Steiner-Davion's child becomes the new Star Lord.
So, breaking this down, without the harshness of the Pentagon cluster, you never see the crash into barbarism that gave Nicolas Kerensky an opening to remake the society. The Pentagon worlds just become a deep Periphery oasis of safe and settled space that there are some rumors and legends about, but they likely never really play a role going forward.
Meanwhile the Inner Sphere unfolds more or less the same, the biggest difference being the Wolf Dragoons never appear and engage in the conflicts they do. Looking over their history, it's storied, but there doesn't appear to be any critical flex points that derail the events leading to the 4th Secession War, the War of 3039, and the setting of the stage before the Clan Invasion.
While without the Wolf's Dragoons and their blood feud with the Draconis Combine in the 4th Secession War things get more complicated for the future FedCom, it's not actually enough to really derail Hanse and Katrina's plans. They still successfully wreck the Capellans and grab up the worlds they need to create the Terran Corridor between the Lyran Alliance and Federated Suns. The War of 3039 unfolds pretty much identically as well, with an effective stalemate between the FedCom and Draconis Combine.
So the Inner Sphere where this POD really starts being felt looks more or less like this:
Where things get fuzzy is starting in 3050 when the Clan were SUPPOSED to invade. Instead the question becomes does Theodore Kurita choose to pass the feud onto the next generation between Hohiro and Victor, or choose to continue to fight Hanse. My suspicion is he would have let Hohiro strike at Victor, and aside from raiding between the two Great Houses, relative peace would rule in the Inner Sphere through the 3050s and 60s.
This has some SERIOUS impacts on the timeline. Allowing Victor a proper military tour of duty before he then returns to learn the finer arts of statescraft from Hanse and Melissa (and Hanse likely doesn't die from a random heart attack without the stress of the Clan Invasion) makes him a much better ruler and more competent. Hanse surviving also means that Katherine likely is found out in her shenanigans and is tempered by Hanse and Justin (who's also likely not assassinated due to the less opportunities available again due to lack of Clan Invasion). This likewise means that the Cappellan Confederation continues its decline under Romano Liao who continues to hold the thrown and is not retributively assassinated thus allowing Sun Tzu to take the throne.
Meanwhile the Free Worlds League continues to be undermined by ComStar due to their replacement of Thomas Marik with their agent.
Hanse and Mellissa likely arrange a marriage between one of his children and one of Candance Liao's children (they both had four kids after all, lots of possible combinations). Between non-heirs though, as factions within their nation would never want a mixed Davion-Liao to be in line for either throne. However, the two families were already quite close (Hanse and Justin were close friends and confidants, and Kai and Victor were best buds growing up), and Candance's main concern was keeping St. Ives independent and safe, and Hanse would respect that out of respect for her, but a marriage alliance would create a strong formalization of the already existing status quo. In the long term while the St. Ives Compact would never be part of the Federated Commonwealth, they would easily be willing to support a scion of the FedCom in a claim as a new Star Lord of a renewed Star League assuming there was a strong degree of local rule (which is highly likely the scheme that the FedCom would adopt if it ever came to that point).
The 20 years of relative peace in the Inner Sphere that the lack of the Clan Invasion brings also just allows the Federated Commonwealth to continue to grow stronger. Economically the two halves would become more and more tied together, and another generation of children would be born and grow up identifying with the Steiner-Davion line and the Federated Commonwealth rather than as members of the Federated Suns and Lyran Commonwealth respectfully.
Sometime in this period Hanse likely offers to treat Thomas Marik's son for leukemia on New Avalon in return for softening his stance with the Federated Commonwealth. This provides an initial diplomatic opening that begins to soften the anti-FedCom alliance's hardline stance.
At some point after this the Capellans under Romano are going to try something. Look, she's crazy and wants revenge on Justin, Hanse, and everyone. My guess is probably somewhere around 3060 she launches what amounts to a suicidal attack on the Federated Commonwealth over the objections of Thomas and Theodore, though with Takeshi's tacit blessing.
This war is going to be brutal. The Free Worlds League would declare neutrality, due to Thomas' prioritizing his son over the alliance, but also because he'll see it as unprovoked aggression and madness on her part. This will piss off his ComStar handlers, but even they will likely see the conflict as doomed from the start due to Romano's madness, as such they likely try and make the conflict as short as possible too since it's not a good time to try and break up the FedCom.
Meanwhile, in the Draconis Combine, without the pressure of the Clan Invasion to create the circumstances to resolve the feud between Theodore and Takeshi, House Kurita is at each other's throats. There is a massive divide in the military and nobility between those who support Theodore's reforms and those who support Takeshi's traditionalism. Come Romano's War, Takeshi (who always was willing to fight Hanse and would see aiding her as the honorable thing due to their alliance) would likely commit his more traditional units to the initial war efforts, likely overriding his son and deploying them even though Theodore would object seeing how the last time the two parties went to war it was a stalemate and not enough had changed since then, and in fact, the FedCom had only gotten stronger and more integrated, and the Capellans were not well organized nor competent, due to Romano's constant leadership purges of anyone she suspected of being a spy (which was everyone, ESPECIALLY those who were competent).
Note all through this Rashalhague is sitting there noping the fuck out. They, like the Free Worlds League, are going to stay neutral in this conflict. This provides a massive buffer zone for the Lyran half of the FedCom, which allows Hanse and Melissa to redeploy troops that would otherwise be on the FWL and Rashalhague borders to bear against the Caps and Dracs.
It's not a one-sided war by any means, but in the end I think the results would be that Hanse and Candance decide that Romano no longer can rule the Capellan Confederation and basically launch an invasion with the ostensible objective of placing Candance on the throne, with an agreement that most captured worlds will remain with the Confederation.
Meanwhile while Theodore will honor his father's commitment to the war effort, he'll likely play things... subtly too. He'll let the traditionalist units take the brunt of the invasion into FedCom space, where the FedCom's newer tactical doctrines will chew them up and spit them out, effectively breaking much of Takeshi's support in the military, and removing more obstacles in the way of further reforms. With the FedCom focused on the Caps, he knows that any counterattack on the Draconis Combine is going to not be their focus, and so keeps his newer units in reserve defending, where they tear up any counter offenses the FedCom throws at them. This pretty much ends the feud between the two with Theodore as the decisive winner.
At the end of Romano's War you'd see the Capellan Confederation gutted and Candance installed as Chancellor, folding St. Ives back into the whole. The Capellans for the most part would now be an ally of the FedCom, still independent, but the damage wrought between Romano's War and her constant purges and tyranny will take generations to repair. As such, they're in effect out of the game, and remember, there's a marriage alliance between the now ruling Liao house and the Steiner-Davion house. This is a MAJOR change in the status quo of the Great Houses.
As this point both the Draconis Combine and FWL likely realize that in the long term they've lost the Great Game. Unless something dramatic changes, the FedCom will end up devouring them piece by piece in the long term. ComStar is also panicking but they don't have the military might to oppose them, assassinating people isn't going to be enough, and long term destabilization plans might work, but those will be to slow.
Projecting this far out gets hard, and really into the realm of Fan Fiction. My personal take would be that Theodore sees the writing on the wall and rather than lose the uniqueness of the Draconis Combine and plunge the Inner Sphere into a true 5th Seccession War that would be nessesary to either destroy the FedCom or allow it's conquest, he goes to Hanse and makes him an offer: we'll make it a fait accompli, have Victor marry his daughter Omiko, declare their child will be the first Star Lord of the Kurita-Davion Line, and go about building institutions to put that in place.
Publically everyone gets on board, save the Periphery States which start looking nervous, but the FWL knows it could NEVER stand up to the might of the combined forces of the Dracs, FedCom and Caps, and Thomas is an idealist who thinks he would be able to have strong positive influence on a renewed Star League, probably going so far as to offer the Knights of the Inner Sphere as an honor guard for the new Star Lord once he is born.
So there would be ANOTHER massive wedding on Terra around 30 years after the 4th Secession War. Victor Ian Steiner-Davion to marry Omiko Kurita. It would be a WEIRD mix of Shinto and Catholic ceremonies... but this is where things turn... dark. The radical arm of ComStar would not be able to pass this opportunity up, as the only way of preventing this new Star League from forming would be, well, to kill everyone at the wedding and blame it on non-ComStar fanatics (like Draconis traditionalist who had considerable secret societies that pushed back against Theodore's reforms and would be utterly outraged at his betrayal by marrying Omiko off to a DAVION). However, ComStar would underestimate two critical people in this plan, Precentor Martial Focht and Precentor Derion Mori, neither of whom would really want or go along with this "kill them all" plan. As such shortly before ROM acts to decapitate everyone, factions of the ComGuards loyal to Focht show up to escort everyone to safety, meanwhile ROM compromised ComGuards and ROM Mech units would launch a desperate attack. It would be a bloody fight, and some of the notable people would end up dying (let's say both Hanse and Theodore get killed protecting their children as does Takeshi and Candance, as well as a smattering of other high level players). The rest however make their escape, and Victor and Horito swear blood vengeance on those in ComStar who killed their families.
Thus begins the ComStar Purge. Thomas Marik is likely killed upon his return home by the Master and the ComStar fanatics primarily use the FWL as their base of operation even as they are systematically being purged from the rest of the Inner Sphere. It is an ugly, nasty affair, more akin to a war against a highly organized and sophisticated terrorist group than a proper nation state. However, due to parts of ComStar splitting off to support the Star League and the outrage that even factions that normally would oppose the Star League feel towards this (IE, the Draconis Combine conservatives would see what was done to Takeshi and Theodore as much MUCH worse than Omiko marrying a Davion, after all, she was never in the line of succession ANYWAY). Within a year of their marriage, Victor and Omiko announce the birth of their firstborn son, Kitsune Theodore Kurita-Davion, and what remains of the Knights of the Inner Sphere come to Terra to swear to protect and serve the boy. Even as the terrorist war against ComStar continues.
Howver, by the end of it, ComStar is purged and folded into the Star League's foundational institutions. The conflict though does provide a unifying effort for the new Star League to be birthed in fire by and bury old grudges. A fairly long period of peace and rebuilding would commence, especially in the now shattered Free World's League. In about 3080 Kitsune takes the formal mantle of Star Lord, though the position has little direct power at the time, with the real power being held by the Star League Council which is made up of representatives from each of the Great Houses. How things go from this point are even less able to be told... a new golden age for the Inner Sphere, or a rapid collapse of the Star League as the generation who built it fail to establish strong institutions and transfer of power to the next generation?
But in the end, one thing would be undeniable. Katrina Steiner and Hanse Davion, by not seeking the title of Star Lord themselves and instead seeking to build alliances and institutions, would end up winning the Great Game for their descendant, at least for a time.
