I talked to the two novices working on Abrax's plant gathering task - one says so and so has gone missing but he is sitting a few feet away; am I missing something?

On my aborted first playthrough, he remained at the campfire throughout. On the second (current) one, he was there until I received a task to check on them from Abrax - then I found him in the forest. The first playthrough was as a bandit - perhaps he actually only dies when getting mage-only tasks?

That should be correct. He disappears right after getting the respective quest ("The test of Master Abrax") AFAIK, so only in a Mage / Order playthrough. I don't remember Sirius talking about Krayban's disappearance when playing as a bandit, though.

Krayban speaks too - but when you offer help, he replies that he needs to do it himself. I think too that it is a bug - basically something minor that was overlooked - he probably shouldn't say that so early :)

I'd say that Krayban and Sirius talking about their assignment from Abrax is OK (even before you join anyone), but if Sirius talks about Krayban's disappearance before you taking the quest and Krayban actually disappearing, then that's most definitely a bug. :P