The Tombstone makes no sense
Ok, Doc's tombstone exists, because Doc dies, because Marty isn't there to save him. In this reality, the ravine is called 'Clayton Ravine', right? Why is that?
Think about it.
WHY do they call it the 'Clayton Ravine'? Because Clara died there.
If Clara died, _HOW_ can her name be there in Doc's tombstone?
A reality where Doc and Clara both die, shouldn't have Clara's name in the tombstone, because Doc wasn't at that location to see her to save her. Why would he go there alone, if Marty isn't involved? The whole reason why he's there, is to try to see if his train idea could work. He doesn't get this idea without Marty's involvement!
Is this movie trying to tell us that Doc - for SOME reason (?!) - just happened to go the same railroad bridge location at the same exact time as he does when Marty is involved, and saves Clara, but yet can't save himself from the bullet, so ..
Can anyone tell me how this makes sense?