@PsihoKekec has beaten me to it when it comes to several points already. So I'll touch on those he hasn't.
Which it would be- this isn't the Russia of the 1940's. Russia in WW1 was much more backwards in its infrastructure. Plus, this isn't the mechanized Heer of WW2. And given the troubles they went though, it's doubtful that the German army of WW1 would be able to pull off the same job quickly when they're slower moving and are operating in even less of a developed Russia.
Furthermore, the idea that leaving the French off for later would be foolish. While the Sheliefen plan did fail in knocking out Frace from the War, it did ensure that most of the resource rich and industrialized parts of France ended up in German hands throughout the war.
And on a related subject-Belgium. And the British. As of 2014, we have a letter from King George to then Prime Minister Loyde George, basically telling him to find a way to get the Empire involved in the war against Germany. To make a long story short, the British were ALWAYS going to get involved in the war, regardless of if its people wanted to or not. Germany was too much of a challenge to them economically and becoming too powerful for the paranoid British to allow. Belgium was at best a convenient excuse.
On a final point- why are we picking Hitler? No seriously, why
Hitler? He hated Imperial Germany and its leadership for not being raging manics like him and being nobility.
Also, the idea that Hitler would be able to blend in and impersonate whoever he's body jacking is laughable. He doesn't have the knowledge when it comes to the sort of environments or person. People would notice something is off and there's a fair bet he ends up in a sanitarium.
Which is where he should have been at in the first place in our timeline.
Plus, Hitler had little actual military tactics or planning skills- most of his successes in WW2 came down to dumb luck, audacity and stealing other people's ideas and claiming they were his.
If the objective it to have the Central powers win WW1, there are a lot better choices for the job. Hell, give me a week to prepare and I could a better job.
