There's a problem that arises when you decide that you want to set the story in a specific setting (in this case, a non-Nazi Germany having come to terms with the Allies in some manner) and that you want to reach this point in a plausible and logical way. The problem is that from any given start point, there are essentially two ways of proceeding.
First, you can let the progression from the POD proceed in a logical and organic way, with the various decisions made developing as they will. They might not turn out as the decision-makers intend, but they go however seems appropriate to the author. That can lead the history to go in unexpected directions, and it's a bit of a voyage of discovery. This is how people make decisions in real-life. They don't get to see what the outcome will be before they make their decision. They know what has happened, and they have the information available to them, and they know what they would like to happen, but they don't get to see what the outcome will be until later.
I once wrote a TL, kind of set around The Troubles in Northern Ireland in the mid 1970s (it started to sprawl rather, but that was the fundamental issue). I chose not to decide what the destination of the TL was, and just wrote it up, day-by-day. People made decisions based on what they knew, and then other people responded to these decisions, and it bubbled along in a somewhat confused manner. The story was the journey. To take one example, the Uganda-Tanzania war broke out during the course of the TL. I had no idea how this would turn out, or how it would impact things elsewhere, but the war developed as it did, with events impacting down the line. One unexpected consequence of this was that a young press officer for the CEGB ended up out advising Nyerere on possible electricity developments for Tanzania. It was a natural part of the journey, and that was fine, because the story was the journey.
The story will meander and go to unexpected places, and you can't predict what the outcome will be 200 pages later.
The second way of proceeding is to decide what the setting is you want, and write the journey from the POD to get to that setting. This has the problem that the decision points are forced towards achieving a certain end. It's one of the big failings of What if Gordon Banks had Played. Every decision point was angled towards achieving a certain end, and in places, it showed. Those factors opposing that direction of travel sometimes made some very implausible decisions, simply to ensure that the desired end result was achieved. People started to act in ways that seemed heavily out of what we know of their character, and the journey wasn't entirely convincing.
SSGB, Fatherland, Man in the High Castle, and so on are telling a story in a particular setting. How that setting arose isn't actually that important for the story. On a more humbler level, it's the approach I adopt in Six East End Boys. The story requires a certain setting. That setting is a given, and it's where the story starts. There's a bit of explanation as to how the setting arose, but it's essentially a hand-wave. It's one that, given the time, I could develop the journey from Thatcher dying in the Brighton bomb to the 30 years later start point of the story; things might be different in superficial ways, but that journey could be told. But that wasn't the story I wanted to tell. That would be a separate story entirely.
It's entirely what you want to write. If you want to write about the journey, then it's likely to be a plausible story of the journey (or less plausible, but that's a function of the quality of the writer). The end point (if there is an end point) is likely to be as much a surprise to the author as any reader. To take the example of my Troubles story, that ended at the end of 1976 with the Green and Orange Balaclavas discussing a possible cease-fire cum agreement with the British Government, with Paddy Ashdown effectively playing the role Mo Mowlam took much later in OTL. I don't know if they ever came to some agreement, or if the difficulties proved too hard to overcome. That would be another story.
If you want to tell a story in a very specific setting (such as a non-Nazi Germany having reached an agreement with the Allies), then concentrate on that story. The journey is not important to the story.
If you want to tell a story in a very specific setting which requires unlikely events to achieve, and to also tell the story of a plausible journey to reach that setting, then I hope you're immortal, because that is a very tall order.