Plenty of area of agreement I think.

I just don't think the NSDAP would have been defeated even if the SPD and KPD somehow fully united (I probably have as much knowledge of the history as you do, or less). Fascism doesn't work like that, it would have just continued to boil under their thin coalition until eventually they would have to put it down forcefully. Just like I don't think beating trump in a single election will defeat the fascist movement he represents. Whoever it is that's leading the opposition has to take (likely un-democratic) action against them if they really want to put it down, and honestly I don't know if it's a good thing or a bad thing that Biden wont cross that line.

Revolutionary movements generally don't fully resolve until the conditions that seeded them change, one way or the other. That's why it's important that whatever coalition that forms the opposition is serious about addressing them, and in my mind simply having the coalition isn't enough.