The movie did address the alteration of the 2015 timeline in a deleted scene where Biff fades out of existence. In the final cut, Biff is seen collapsing in pain shortly after returning to 2015 in the Delorean. Originally, this went on a bit longer, showing Biff literally disappearing the same way Marty almost did in BTTF1. The reason was because he'd returned to a future where he didn't exist anymore, because in the timeline where he is rich and powerful, he ends up getting shot (as speculated by Bob Gale) by Lorraine in the 90s.
Wow I had no idea! Thank you. It's great stuff !
Whatever ramifications caused by Biff's actions that spilled into the year 2015 wouldn't necessarily be noticeable, at least not in such a way that a movie with BTTF's tone and running time restrictions could have realistically conveyed to the audience. And even if Biff giving the almanac to himself in 1985 did make Hill Valley 2015 a significantly different place visually than it was when Marty and Doc first saw it, they would never have noticed it because the timeline would have changed around them and they would only have memory of the new timeline (as Doc explains when Marty protests leaving Jennifer and Einstein behind when they realize they have to go back to 1955). Obviously, it doesn't all add up under close scrutiny, but it works in a movie logic kind of way.
It's not really possible they couldn't notice change. If they didn't notice the future changed, that means they had knowledge of what happened in the new past too. If the future becomes their reality that means the past of this future also his (in my opinion). If, for exemple, they don't notice in the future that, Biff is all mighty, why would they in the past?
The only thing possible is, as you said, that they didn't see anything different, not that they couldn't. And that's actually a good explanation, since the second they got Jenifer out, Biff came back. They couldn't see if George was dead or something.
And now that you mention it, since Biff was affected by the changes, shouldn't Marty and Doc change too ?
As for Jennifer, I'm not really sure about that but... Letting her in an alternate timeline and going back to the past to change it...
Does that mean there aren't alternate timelines but only ONE timeline and no matter what if something changes it replaces everything ... ? O_o
They can only travel to one timeline but I wouldn't say the others just get erased or whatever ...
Now what's weird, is that if Jennifer is affected by changes in the past, why aren't Marty and Doc when Biff changed it ?
Their past selves had pretty different lives (and probably never time travelled).
We know that a past self affects the future self... So we have to guess that the past Marty and Doc aren't the real Marty and Doc but the ones from another reality, but doesn't that mean Marty also created an alternate timeline in the first movie, and didn't really go back to his ?