I don't think Doc was suicidal.  He was completely confident that the time machine would work.  Imagine the look on your face when you build something and it actually works.

Here’s a theory I was thinking about recently: I back to the future 2, Doc taking Marty into the future to help his kids was irrelevant. Being a scientist, Doc brown would know that anything you change in the future has no bearing because the future is never set, meaning the future can only be changed in the past. Doc knew about Marty’s accident so I think he took Marty to the future to let him get a glimpse of things going wrong in hopes to get Marty to change his way. However his plan backfired when Biff saw doc and Marty talking about the almanac and Biff ruined Docs plan, until Doc let it slip about Marty’s accident in back to the future 3.

Honestly, the idea of the original actor for Marty being the original, and (in-universe) the actor changes because of the change in the timeline is actually a cool idea.

That the Delorean can't make paradoxes very conveniently explains why it wouldn't start when Marty was trying to make the lightning run: it had to start exactly when it did for him to reach the cable at the exact moment it was energized by the bolt of lightning.

Except that doc didn't know duplicate Marty was coming early, this wasn't in the note, this was something Marty did himself after he reached the starting point. At no point do these duplicates interact so the danger would have been minimal anyway.

George would have trouble knowing that Marty was a time traveller if Eric Stoltz was the original Marty.

Doc's first exposure to time travell is in 1955, by the time he "invents" time travell in 1985 he already knows it is possible

the theory that Doc murdered Marty doesn't make sense; he would be smart enough to know he had to allow the original Marty to go back in order to come back to avoid a paradox in the space time continuum.

Back to the Future and The Goonies take place over the same weekend in 1985

For years I didn't even know there were two Jennifer's

12:15 Biff doesn't use the almanac to change the sporting events themselves, he uses it to always place his monetary bets on the team that's going to win. He doesn't change the events, he abuses future knowledge.

The thing is that Marty upon returning to 1985 is simply observing future events playing out in an endless loop.

This explains why Marty instinctively knew to contact doc's 1955 counterpart upon recieving the letter from doc after seeing the delorean  struck by lightning  knowing that doc's 1955 counterpart was the only one capable of helping him return to 1985....of course in the process of uncovering the delorean  they  both discover doc's grave from 1885 and the focus shifts to Marty rescuing doc instead of simply returning to 1985 as originally planned.

I like the idea of the delorian being incapable of creating a paradox. It is constantly broken for plot reasons. Maybe not as a plot reason but because the traveler has to influence the timeline in specific ways before it can function again.

vulcan literally did mean something in 1955
1. a roman god
2. a hypothetical planet between the sun and mercury that fit with old theories about the solar system. those theories would have fallen out of favor by 1955 but it still showed up in science fiction from that era

Calvin Klein doesn't grow up to look exactly like his son; his son grows up to look exactly like Calvin Klein.

There is no duplicate Marty. He watches himself go to 1@55 and then there’s just him. No different than when he watches himself perform JBG in Part 2

The theory that doc made multiple attempts to save Marty collapse when you have two Martys running around 1955.
If he missed, he would see himself missing and there would be multiple versions of him trying the save at the same time

The Lockard theory also makes sense from the perspective that BTTF Part 2 is split into two major parts that both deal with fixing tragedies. As for Marty not knowing about his changed family past until it's explained to him, I think an unspoken rule of the franchise is that time-travelers are immune to other people's changing memories from the changed past.

2:20 Doc already knows that the car works. He saw it work in '55. So he knows the car will disappear when it reaches 88mph. He's not suicidal.

Honestly, the sequel to the first film should have begun with George confronting Marty one day about realising that Marty was Calvin Klein.