Originally Posted by Michael J Fox is Canadian
The future (in BTTF) is similar to the past in the sense that it can not be changed without time travel; look at 2015; doc clearly maps out what will happen on that day and everything happens on schedule. It's not just a possible future, it is part of the timeline as the same events occur until they are changed by time trave; the newspaper of the next day still exists as do the almanac and biffs cane. also the fax of marty being fired; this functions similar to photos and newspapers that we see change; they change when the junction point causing them changes; All Marty's life (up to 2015) he has the flaw of not being able to back down when being called chicken. The events of the old west make him start to believe he shouldn't care what others think; any combination of Seamus brother Martin getting killed by being like him, the fact that Doc wouldn't have been stuck in the old west if Marty backed away from Biff, and that using logic over emotion (cheating in the duel with Buford) prevails leads him to that conclusion. The car accident was a critical point in Marty's life which led to the future we see in 2015. But in that timeline Marty did not have the events of 1885. Once Marty avoided it (again showing him that backing down clearly was the better choice), the fax erases.
Now there's many reasons the fax could erase. One being that if in that situation again in 2015, Marty wont reply to Needles calling him chicken but also that he does not give up on his music (as the car accident does so).
It is difficult to get a good gage on what you're saying because the only time travel during the films or games that we do see on screen which doesn't end up changing the timeline is Einstein going one minute into the future. But the 2015 experience is a good indicator on how the timeline plays out.
So in episodes 2/3 the delorean still exists in the FCB timeline for the same reason the almanac and cane from the future still exist in Hell valley; they weren't erased from existance.
Its true the future is shown to be just like the past in BTTF2...with events unfolding the same way until they are changed by the time travelers intervention. Okay...maybe that reasoning is a bit flawed. But at least in the case of the past, it was made amply clear in BTTF1 that the past wasn't just another room where you went, made changes to, and returned to the present like nothing happened. The causal link between past and present is clearly illustrated in BTTF1, with virtually EVERY action caused by Marty's presence in 1955 has some repercussions in 1985...from the smallest ones (running over the pine tree causing the mall name to change) to the biggest ones (George's confidence which translates into the McFly family's success).
But this causal link becomes a lot less clear with 2015 though. Because, every second Doc and Marty spend in 2015, they change not only the future, but also the PAST. Because both Doc and Marty are from 1985...every action they take, which wasn't taken in the previous timeline, affects 1985 as well, and consequently, 2015 should logically be changing, even imperceptibly if nothing else, every second as well. Everything 'our' Marty sees in 2015 is something he hasn't seen 'before' (in the previous timeline), and so he his very presence in 2015, by changing HIS timeline and therefore the timeline of 1985, should be changing 2015 as well! But instead, 2015 seems to be like this parallel universe or possible future, which Marty and Doc can observe and interact with to change stuff directly, but which has seemingly no causal connection to them.
The original 'Number Two' script, despite its flaws, actually did attempt to illustrate this causal connection between 1985 and 2015...when 1985 Jennifer pricks her finger on a thorn, we see 2015 Jennifer experience some residual pain from that. Now personally, I'm glad this 'pain transference' thing didn't make it to the finished film, but at least it illustrated the fact that the future Jennifer was causally dependent on the present Jennifer, and her past was changing in some sense with every action of Jennifer's in the future.
You do make a good point about the newspapers and the Almanac. In fact, that's one of the peculiar points about causality in BTTF. The information contained in objects from the future change based on the time traveler's actions in the past (relative to the origin point of those objects), but the objects themselves are never erased, even if the causal chain which leads to their presence in the time traveler's hands, is erased. So, the 'GEORGE MCFLY MURDERED' newspaper changes into the 'GEORGE MCFLY HONOURED' newspaper when the Almanac is burnt, which makes perfect sense given that the future has changed...but the newspaper itself still exists in Marty's hands in 1955, despite the fact that the chain of events which LED to the newspaper's presence in his hands has been erased. Which makes me wonder...could the same apply to the Delorean? Can the Delorean still exist, even if the chain of events which led to its creation has been erased?