We also know that Malak is part of Tarquin's inner circle, which makes him being an ally of Nale to be low.
Looking at Soul Eater in the BoVD, where does it say that their victims can never be Resurrected? Yeah, they throw around negative levels like crazy, but IIRC dying by negative levels is a "Death Effect" and while you can't be brought back by Raise Dead, but you can by Resurrection from that, which we know Durkon can cast. Also, Soul Eater gives no continued spellcasting (as it's pretty clearly meant to be a power-up class for monsters), so to be any good at it Malak would have to have traded out a LOT of spellcasting power.
Yeah, there isn't a Linear Guild cleric we've seen yet, and we haven't seen what Durkon is doing, and while there's an off chance it's Malak, I'd say it's unlikely, and being a soul eater is very unlikely. Him being a vampire is another theory I have heard, but haven't we seen him active during the daytime?