I got a better question
How was she his wife on the tombstone when they just met three days before he was killed?
I can think of a reason why Marty would remember. Clara even though she didn't die
He was outside of time when doc went back so just like Doc himself was unaffected by changes to the timeline neither was Marty
When he returned to 1985 she would no longer have died but he would still remember her and wouldn't even realize that the name of the cavern has been changed
And the simple answer would be that doc was in that position to save her without Marty being there
In fact, Doc could have been in a position to delay her and have the incident. Just never happen at all
He could have met her on the road
But it doesn't answer my question
How the hell was she his wife when they met three days before he died
Another question I would have had is why is Doc Brown's logic so flawed
He acts as if leaving her behind is the right thing to do
Considering they know that she was supposed to have died, the only choice of action that you have is to take her into the future
Heard some people say that she responded badly but all he has to do is actually show her the time machine
That's not that hard
He doesn't even have to tell her what he's going to show her. He just has to say I have a surprise for you and something you need to see
And then explain to her that she was supposed to die in that ravine. So her staying here would have devastating effects on the timeline when they came home
Every single thing that she did in her life would change the future and it would be compounded horribly if she had children
The only choice that a scientist would have at that point would be to take her with you
If she refused to go you would have to kill her. It's that bad and the movie acts as if leaving her behind was an option at any point
It never was and they would have come back to a completely changed 1985 had they left her there