Okay. First, it's worth remembering that the writers' sloppy and often contradictory handling of time travel in the show is something fans have been complaining about ever since season one. The writers sometimes get it wrong. There are things that don't fit. So, with that in mind...
No one's memory changes. Every time Flash steps out of his reality (A) and creates a divergent reality (B), he always retains his memories of reality (A). That couldn't happen in a single reality because the instant you change the past, you'd change your memories too. Reality (B) would be all there ever was, you were always a part of it, and that's all you'd remember.
So for example in the 'flashpoint' divergent, Flash still remembers reality (A). But there's no change to the memories of the Henry and Nora of the Flashpoint divergent. That's the only reality they've ever known.
Reality (A) is still out there, still coexisting. But Flash doesn't get back to it. He ends up in a third divergent (C) which is similar to (A), but with differences.
Wrong again. Flash's memories were changing in Flashpoint. It just took a while and was happening gradually, but it was happening. Savitar's memories kept changing too. The Legends' memories get changed lots of times. They all even noticed it happening.
After Flashpoint, Harry and Jesse came back from Earth-2 and noticed that things were different on Earth-1, showing that the new reality was right where the old reality used to be. If it worked like you say, Harry and Jesse would have come back to the original reality instead of the new one. Instead they found the new reality there, because it replaced the old reality instead of coexisting with it.
Yes, that's one of the things they get completely wrong.
If it was a divergent reality, Flash's memory wouldn't change at all.
If it was a single reality he'd ONLY remember reality (B) and have no recollection of reality (A), because (A) never happened.