The new 1985 timeline remaining much the same, despite huge changes made by Marty McFly in the past

Well, it couldn't be too different. Or it would have been unrecognizable to Marty and the viewers.

I guess that they just liked that house.

We don't really know much about Marty's brother. There might be several reasons for why he still lived at home (as if that was a bad thing anyway).

And why is it not likely Marty should have known Doc or dated Jennifer?

His parents aren't "wealthy" in the new timeline, even if they're better off. George isn't yet an author when Marty returns, but George is now in a supervisor position at the same company he previously was employed at with Biff as the supervisor. So his salary was better as represented by the nicer furniture, clothes, cars, etc., but not enough to make him wealthy.

I lived at home until I was 26 when I bought my first house and got married, so Dave living at home isn't far fetched. I never once lived in an apartment.

I don't understand how anything would have changed with Marty and his girlfriend. He went to the same school where he met Jennifer. I don't see how his parents having a few more bucks and his mother being sober would change Marty's attraction for Jennifer.

Honestly, I think too many people are overthinking the plot of the story which features a time-shifting story that defies the laws of physics. I mean, if we're going to pick nits, why not pick on the fact that time travel is impossible?

All fair questions, but here's a couple possible answers:

We don't know much about Dave. It wasn't uncommon for guys in there mid twenties to still be living with their parents. But he might not have been. We learned in Part 3, that they were all going to brunch. So maybe he had to go into the office for a few hours, then went to his parents house to wait for them to get back from their tennis game.

Maybe since his parents are now in a loving relationship, and Lorraine isn't drunk and depressed, they actually gave a crap about how their house looked. So they kept it clean, and bought nice furniture, and decorated it well. Also helped that George obviously had a better job, so there was more money available for them.

Perhaps during the week in 1955, Marty tells Doc about how they met, and why he became friends with an older, slightly mad scientist. Doc would use this info to ensure that they still became friends.

Coming from a much more positive and loving family, might have changed Marty's upbringing and personality. But Marty was already a pretty sharp, and likable guy, and Jennifer was still smoking hot, so they could still be together. Plus Marty now has sweet truck.