If companions leave adventurer's party due to breach of agreement or other reasons will they set off to their hometown and can they be tracked down? If they stumble on a night creature's lair can they be attacked by it?
Rephrase:
If companions leave adventurer's party due to breach of agreement or other reasons will they set off to their hometown and can they be tracked down and murdered?
It's not clear from the devlogs if that will be possible. If a single person leaving your party is tracked as an "army" (like the player and their companions are) then they should move slowly over the world maps and it ought to be possible to track them down. It seems more likely that they will just teleport back to town (or wherever they came from), but it just depends how it's been handled, so we need an answer from the Toad on that.
As for them meeting a grisly fate
not perpetrated by the player, that is almost certainly a 'no'. It's known that megabeasts/night creatures/whatever else have not been moved over to the new mechanics, and that they don't have their worldgen activities extended to regular play. There has also been no mention of armies interacting with critters at all during play yet. Naturally that's all almost certain to go in eventually, and it wouldn't require a huge rewrite to make happen, but the current goals seem to be to finish up with everything that has been roughed-in so far, and then go for a release.
I would speculate that a lot of relatively low-hanging stuff like this (things that happen in worldgen but can't happen during play yet, and off-screen fighting etc.) could be the next set of features Toady decides to implement after this release, purely because it is relatively straightforward to head in that direction now that the world is no longer stagnant post-worldgen. Then again, there are a pile of other interesting things from every possible direction development might take.
On a seperate note, I'm looking forward to the devlog mentioning elf-tree-related bugs. I would put money on trees piercing the underworld within the week. All the good features in DF accidentally pierce the underworld at one point or another.