Warning: all of the following is speculation, of course, but as you said, there are lots of open questions, and speculating helps pass the time until ToB ships.
Manveru, do you mean "they" as in the group in the end movie? If they are indeed Bhaali cultists, then maybe daddy gave a select number of his most loyal priests some information about who and where his Children are before he croaked. Alternately, it seems that word about the PC's origins has spread to some degree already, and it'd be logical for old Bhaal-worshippers to be paying attention to such rumors and other signs.
My "Wanna bet we'll prove him wrong?" quip was aimed at the fact that our intrepid hero hasn't exactly complied with the various badguys' (Sarevok, Bhaal, Bodhi & Irenicus, Firkraag, you name 'em) plans so far.
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Even if the PC dies in the end (which I hope won't necessarily happen), I think she'll be sure to stir up some serious trouble for the Mystery Men.
As for the role of Gorion and Elminster ... I wouldn't call it "taking control" of the Bhaalspawn, but rather an attempt to guide the Children (PC & Imoen) away from evil and prepare them for the inevitable conflict with the dead god's will and their own siblings, by teaching them strength of mind, will, and character. But the result, preventing Bhaal from returning (through these two offspring, at least), would be the same.
And yes, Elminster's letter to Gorion seems to foreshadow a greater conflict than the one in BG. Personally, I hope that Gorion's role and background will receive some more attention in ToB, but I could do without seeing Elminster again. Or Drizzt, for that matter. If they
must involve one of the "signature characters" of the Forgotten Realms, why not bring in an equally powerful but less-overused one like Khelben "the Blackstaff" Arunsun? He, too, was known to Gorion, and I think it would make for a fun conversation if Khelben was to remind the PC of the incident with his cloak (if you don't remember what I'm talking about, talk to Theodon in Chapter 6 of the original BG -- that little anecdote had me in stitches when I first played the game).
Re: Viekang (the "self-destructing guy in Trademeet") ... I
think Bioware has confirmed that we'll learn more about or meet him again in ToB.
"Beware of the blindness of those who would follow, and the damnable lure of those who would lead."
- Tamoko
"Mmm? What's this? You gots hammer? Bhaal once drop hammer on big godly toe. Jump around and swear for days, he did. Kicked poor me all the way to Baator. Very bad week, that."
- Cespenar the imp