1. He got a letter too. Actually, he got two: one from Valen, and one from Mah'hel (just in case).
He believed it, improbable as it seemed, because the whole idea was just the sort of stupid, self-sacrificing heroics that Jeff would go for. After that, it was just a matter of making sure he was in the right place at the right time not to get left behind.
2. He's a paranoid son of a bitch.
When Sheridan told him to turn back, okay, so while technically his reasons made sense...something about the whole conversation just pinged his bullshit meter. Once he'd followed them into the time field...well, by then Jeff couldn't send him back if he wanted to. Not without killing him in the literal as well as figurative sense.
3. It was an accident. Really. Not his fault the time field expanded while he was recording that transmission from the alternate-future Ivanova, sucked him in, and kicked him out the other side a millennium in the past.
Convenient, yes. But despite what Jeff might think, even Garibaldi's not devious enough to plan that.
Draal, on the other hand...
4. He dreams about him sometimes. Dreams where the Minbari bonecrest and the brown robes melt away and it's just Jeff, like he knew him. Dreams where Jeff actually gave him a choice. Oh, they argue in those--it wouldn't be Jeff if they didn't--but in the end he has an answer for every reason he shouldn't have gone along.
Maybe the universe was listening, because one day he just wakes up and he's somewhere else. Someone else. Someone who remembers being Michael Garibaldi, but doesn't miss it because he's where he's supposed to be, where he wanted to be, at his best friend's side, and nothing else matters.
5. Let's just say it involved a TARDIS and a Chameleon Arch and leave it at that.