Völler's Hitler plan (SPOILERS)

Voller should have minored in geology at universtity. "I would have gotten away with it if it weren't for you meddling tectonic plates!"

In 1939 Hitlers second was still Rudolf Hess. He was depicted later as a mad men, because he insisted on the peace treaty with England, but most likely this was just the Nazi narrative to discredet him.
He was political ousted with the war, as he was not really helpful as non military. But before the war, he was the one with the strongest claim, Hitlers official Second. But not sure, if he would be able to take control. More likely Himmler with his most effective executor Heydrich would have done a silent coup and take control - the SS was by 1939 already powerful enough. Goering of course could have also made it to the top, as he was by far the most popular Nazi by 1939 (he only became a joke after he failed conquering UK with his Luftwaffe, became fat and a junkie). His popularity was the main reason in the first place he was so highly ranked within the Nazi party, as the other Nazis hated him.
If anyone wonder: Goebbels was always happy to be the man in the shadow (as long he got a hot girl). He was more the Machiavellian type.

now youre being stupid. youre just thinking like an idiot like Rhodes from the Avengers Endgame.

Because he's needs for Hitler to have built up the nazi party and Germany. At 1939, he'd be inserting himself into an already built up country. Going back earlier leaves too much to chance.

My understanding was that the device was not a time machine that you could program to go anywhere and anywhen, it was a machine that merely indicated when rifts in time appear.

Yes, spot on.

His plan always had flaws because of his arrogance.
But my guess is he wanted to change two things in Hitler's favour: The Battle of Britain and the march to Stalingrad, change what general Paulus did.

The continental drift thing was a moot point. If the calculations had been wrong, they wouldn't have found a time rift at all. Finding a time rift at the given time and place confirmed the calculations were correct. Besides, Archimedes was found to be wearing a wrist watch - which suggested at that point that Archimedes had himself travelled in time, so why wouldn't Archimedes know about continental drift? As it turns out, he found that wrist watch off a dead Voller, but the very presence of that watch in Archie's tomb meant he had been in touch with the future, and so it was fallacious to assume he couldn't have known about continental drift.

No, his '39 self was about to meet Hitler regarding the V1, which was used during the Blitz to terrorize the British. The V1 was actually a cruise missle, while the V2 was a proper ballistic missile. The V does not stand for rockets/missles, but actually for weapons which could change the outcome of war. V3 was e.g. a cannon.

Anyway, his plan was to replace his 39 self, kill Hitler and reveal being a time traveller who knows best how the war is to win. Hitler was megalomaniac, he did several power plays against his generals which usually ended in situations worsen the situation for the Nazis. Likely already preventing the british army getting rescued at Dunkirk would have killed the moral in Britain had weaken Churchills political standing massively. So a peace treaty could have been possible or at least Rommels North African war had played out drastically different. With North Africa secured, oil would have been secured, no reason to get into Russia that early. Ripple effect.