I didn't think this was a specific timeline, just a discussion of a divergence point. I don't see Napoleon being able to hold on to any substantial conquests even if he defeats every other power's ready forces and if he defeats the nearer enemies whose forces arrived most promptly those farther or more laggardly may choose to preserve their forces. If he tries to conquer any great distance from the loyal French core after the Spanish campaign demonstrated the power of guerilla warfare I expect he'll be up to his armpits in partisans and the other European powers will double down on destroying him. Only if he wins the battles and then stays in France can he have any chance at convincing any of his opponents that he's not worth fighting to the last right now.France was smashed back to its own borders but the TL assume its going to make substantial conquests and defeat every other power, at least their ready forces. Even the proposed greater French empire, if Napoleon suddenly decides his days as a conqueror are done will pose a continued threat to Britain as long as its perceived as hostile and while Napoleon is in charge there's good basis for those fears.
England maintained the blockade for years when France occupied or subjugated those other powers so I think it plain that the expense is not unsustainable. England doesn't need to stop overland trade, it need only stop France from developing naval expertise. I am thinking of a situation where Napoleon is reluctantly tolerated because the European powers do not wish to pay the price of further war, but would be quite happy to have the Brits keeping France kept from any colonial resources even if it means deeming any of their merchants that make landfall in France to be smugglers. Possibly Napoleon can only get armistices from some nations until he has spent years respecting those armistices.The problem with this, other than how you check on what France is building or not is can Britain continue a hugely expensive blockade if France makes peace with other powers? That's going to mean clashes with the merchants of those powers as well as that Britain can't blockade overland trade, albeit that other than by waterway that is still limited.
If France accepts naval limitations as the price of peace that implies inspectors, but I think France only has one inland river port suitable for shipbuilding that can't be observed from the sea. A man with a spyglass on a mast top can see farther than land based guns can dissuade a ship from approaching a port.