You're welcome to try and explain how the soul of a mere human battlemage could possibly power a massive iron god.
Eh...I hate to sound like an idiot, but...it always clicked in my head that Wulfharth's soul was in the Mantella, mainly because it's stated that the soulgem is used on him and at the time it was easier to process in my head that an avatar of Lorkhan could power the Anumidum better than Arctus' soul could. I feel that, in some weird ass way, the Underking's Heart blasting the crap outta Arctus' somehow made him become the next Underking.
In the end, I find both about equally plausible though.
To Ravager: yeah he does, though I don't remember if he stated that in the interview.
edit: Nope he doesn't. But check these pieces of info:
1. If Nerevar didn't kill a god at Red Mountain...why the hell is he "Godkiller" then?
2. According to The Five Song's spiritual sequel, the Arcturian Heresy, eventually people got confused on who Tiber Septim was, thanks to the Underking's placement in another part of Tamriel, as well as other crap. It was as if Septim was in two different places at once.
3. Like I said, Vehk himself even references events recorded in the Five Songs and the Heresy, most importantly the point where he killed Tiber Septim.
...Yes. He was at Red Mountain.
While as always, this is merely my opinion, I still feel it has much merit.