I don't think there's a "canon" theory, on either of these, but here's my take.
On the first point (people in the
BG2 ending), I disagree with Grunker, I don't think that's the Five, if only for the sole reason that a fire giant wouldn't fit at the table.

Just me, though. I'm leaning towards the Bhaal cultists theory.
As for the second point, before BG2ToB even came out, it was my theory that Bhaal had a little hideaway on his own plane of existence where he stashed a bunch of statues of all his Children, and that when one of them died, said statue dissolved into dust. My take on the movie at the end of
BG1 is that even though the PC may have vanquished Sarevok, there are still many other Children out there who will probably come after him/her, at some point. Kinda like a cliffhanger of sorts to say "oooh, perhaps there'll be a sequel in which you have to fight off your siblings!"