Yeah, same with the Elites of course.

: I've also been able to... convince the Marines to fall to their deaths... en
: masse. Apparently, when exiting vehicles, the AI does not consider where
: they'll end up.

Occasionally I've used this as a way of getting Marines eliminated without being blamed for it. I'm thinking of something in the second part of level 2, using the sheer drop down to the sea.

: Also, when I first set up my Cavern Megabattle, I found there were a couple
: of Marines (about two or three, to be precise) that had lingered in the
: Two-Wraith Area. So I would kill all the Marines, and a while later find a
: couple more coming in, inexplicably. I eventually was able to coax them
: into the checkpoint zone and get a new save, however. I was wondering,
: have you encountered anything similar?

No, as I've got a careful set-up routine. But the behaviour doesn't surprise me. In general gameplay, when you head down the tunnel away from the two-Wraith area, I've noticed that Marines can quite easily get left behind if you don't take care to make sure they're following.