Regarding Clayton/Shonash/Eastwood Ravine -
It is possible, though admittedly unsupported by evidence as far as I recall, that Clara Clayton, overcome with grief after Doc’s demise in 1885’, threw herself into the ravine.
I have a theory that might explain much of the seeming paradoxes of Back to the Future, if it weren’t for one of the main points of the first one. In the first movie, Marty interferes with the timeline, and eventually begins to fade out.
Yet in the sequels, we see numerous changes to the timeline that don’t cascade - Biff, after giving his younger self the Sports Almanac, returns to the future in which he’s a bitter old man. These sorts of things would seem to indicate that time-displaced folks act as a kind of anchor - since Marty and Doc are still in 2015, which isn’t their proper time, they’ve anchored it in place, and Biff returns there. Unfortunately, Marty’s fade-out is inconsistent with this.
My only other theory is that changes to the timeline actually take time to cascade - time measured from the perspective of the actor changing events. Thus, Marty has several days to get his parents to kiss at the Passion Under the Sea dance. Thus also, Old Biff returns to find the future unchanged from the one where he stole the DeLorean - so far. Had Marty and Doc hung around for a few days after Biff got back, they may have found things a bit different. Instead, they head back to 1985 - leaving Old Biff’s frame of reference entirely - they arrive in a 1985 where the cascading changes have already caught up.
I can only guess they’re protected from those changes by the fact of being ‘in transit’ in the time machine past the cascade point. It’s also possible that the events just aren’t stable - that the timeline exists in a flux state until the changes are ceased.
One thing that does seem consistent about the series, though, is that time-displaced individuals do not get their memories altered based on changing events. Marty doesn’t “remember” his siblings disappearing, he has to rely on the picture. Doc doesn’t remember dressing Marty in those clothes. Marty and Doc don’t remember the events that led to alternate '85, relying on Newspapers.