Why SNW Chapel might become TOS Chapel: Recovering from an injury/illness that curbs her ability to go on away missions (but still allows her to serve)
Why Spock might owe a lot to Pike: If La'an is lost somehow, Pike might support him, having lost Marie forever and therefore sympathizing (even though she's not dead, it'd feel like a death) - maybe Pike tries to save her but fails.
What if La'an was on the cadet ship Pike rescues kids on, too? He tried to save her but failed - maybe she told him, get the kids first - he goes back for her but she's dead. Nothing says there were no other non-cadets on the ship.
If you think about it, Spock pulls a Pike in TWOK - saving the people on the ship, at high risk and ultimate price to himself.
Hell no. That undercuts both Pike and La'an by making everything connected in yet another case of small universe syndrome. We don't need to know La'an's ultimate fate. It's perfectly fine to still have mysteries.
I'm hoping that Chapel stays more or less as she is. Chapel losing herself could be an interesting individual episode (aka Voyager Extreme risk where B'lana becomes suicidal) but this would a very depressing transition long-term where she becomes a shell of herself. I think they would lose a lot of viewers.
Let's have La'an staying perfectly alive with some promotion or career opportunity away from the Enterprise. She doesn't need to have some sort of ultimate fate. It was a mistake for Batel and straight out of Doctor Who. She can just live on with a new job and possibly pop into Year One if they get it. Please let's not have her die and Spock become his logical self because of La'an - she's a casual partner / rebound. It's a disservice to both of their characters. He's going to have to start transitioning before the end of the show and she's going to be fine until at least the end so I doubt that she's going to die or that this will have much of an impact on Spock.
Pike's accident would have the most obvious significant influencer on Spock but I don't think the dates would add up for that to work? I suspect Sybok or another clash with his parents.