I’m uncomfortable with treating these two events equivalently. So I’ll address the Sarah Lynn question, and ignore the second part.

He would be able to live his life without thinking that he was probably responsible for someone’s death. Not just anyone, either: a friend, a colleague, a former lover, a sort of daughter figure. Someone whose life he made worse by being around her.

The difference it would have made was inconceivably huge.

Here’s what I think would happen.

They’d both see it as a wake up call to get help. Sarah Lynn has already been to Pastiches and advises Bojack against it. They get into an outpatient program that’s actually interested in helping them and not just being an expensive summer camp. Bojack would bring Chloe to Sarah Lynn and ask her to “Tell this girl everything you wish someone told you at that age.” Sarah gets work on Ethan Around as “Auntie Bri” who’s a fun and carefree aunt who has to be reined in from spoiling the kids. It would be bittersweet if Auntie Bri worked as an architect.

Mr. Peanut Butter is doing his shenanigans running for governor. He tries to rope Bojack into being his running mate. Bojack is all “Hell no”. He might even make a public statement that hurts Mr. Peanut Butter’s chances, not that he had much. Maybe it culminates in a duel a la Hamilton. But, something would happen like Todd switches the guns out for bang flags or Mr. Peanut Butter literally shoots himself in the foot.

Bojack does go to the summer home in Michigan in the summer, when Ethan Around is on hiatus. He brings the cast with him, determined not to make the same mistakes he did with Horsin’ Around. He gets more on the ball with repairs with his guests getting on his case. He gets Eddie to help and plays the same stunt, only this time someone’s there to break his fall and tell him what a dumb idea this was. Eddie is mad, but won’t try to kill him in front of witnesses. When he breaks down, someone (Chloe?) comforts him just by sitting by him while he cries out. They part maybe not as friends, but not hating each other either. Also, at some point, Bojack and Sarah Lynn decide that while they’re good to each other, they’re not good for each other and should not pursue anything romantic or sexual.

Hollyhock shows up. Beatrice moves in. Bojack tries hard not to relapse while going out to get milk and having an anxiety attack. He calls Sarah Lynn for help, who brings them milk and joins them for breakfast. Bojack still has a fit over Beatrice’s doll but has Sarah Lynn to help him look for it. Bojack never does FHBA so Hollyhock never meets Miles. However, she could easily meet someone like him on the set of Ethan Around. Bojack tries to get Sarah Lynn to make her see the truth. She tells Hollyhock “Yeah, this guy is just using you. So you use him right back!” The guy leaves her when she gets too demanding, deciding an audience with Bojack isn’t worth it. Hollyhock is depressed and starting to feel effects of Beatrice’s coffee.

Sarah Lynn invites Hollyhock out clubbing saying “The best way to get over someone is to get under someone!” Hollyhock wants to just stay home and drink coffee. She gives some to Sarah Lynn, who takes one sip, throws the cup across the room and screams “Who’s the dumb shit who’s trying to poison my friend?” Sarah Lynn reveals her mom also gave her drugged drinks to keep her thin, give her energy, calm her down, whatever she needed to be to keep dancing. Sarah Lynn wouldn’t be as nice to Beatrice as Bojack was. She’d just dump her in the lobby of a nursing home with “Suck a dick, dumb shit!” Because Hollyhock didn’t overdose, she might stick around but be more cautious. She and Sarah Lynn would have an odd friendship.

Everyone else would pretty much do as in canon.