okay so let's look at the Lawden spoilers again...
http://screenrant.com/iron-man-3-thor-2-spoilers-rumors/3/
By the way, I haven't been following IM3 stuff too closely, (though I will definitely see it in theaters) has he been proven/disproven yet on anything he said about IM3 in that interview?
so as you know the big thing that was bothering me about these spoilers is that he seemed to be referring to Thor and Loki needing to get warriors from Valhalla and that Loki only has access to that and that Valhalla is referred to by Lawden as the "dark world" (which it is definitely not). The thing about them assembling and training an army bothered me too, since the Valhalla warriors already have experience as an army, some probably more experienced even than Thor. So that all didn't really make sense to me. Screenrant assumes the same thing about the dark world = Valhalla on that website link, so we were all taking that lead I suppose, and I think that may very well be incorrect now. We may not see Valhalla at all in Thor 2 and the Einherjar characters listed on IMDB might only be the regular Asgardian soldiers and guards, as they were referred to that in Thor 1.
So instead, I think IF all the Lawden spoilers are correct, that it may be it's more to do with Thor needing Loki's personal connections with the darker side of Asgard to gather and train an army to defeat the various threats. Then the gathering and training an army part makes A LOT more sense as well. Also if Loki is master of magic then he must have a mentor and not necessarily Odin, but that could be a
variation of the role of Karnilla and/or the Alice Krige character. That could certainly work Nornheim into it, from what I can tell Nornheim IS a dark side of Asgard. Or Loki could simply have made some shady connections with the darker forces of Asgard in the process of learning and gaining power with his sorcery, and that's why Thor needs Loki and how Thor is able to gather and train an army to defend Asgard. They have been hit hard and Thor needs all the help he can get to keep Asgard in the hands of Asgardians. (and this may mean a really great rallying cry "For Asgard" speech by Thor, I hope!) . And of course Loki using his dark world connections like this, could certainly leave a door open or two for some crosses and double crosses during the course of the film to keep things interesting with him.
Another idea, is if the Dark Elves have successfully made part of Asgard a "dark world" then there are more Asgardians trapped on that side, and it may be only Loki who is able to break through to get Thor etc, into that part due to Malekith's magic, and then fight off the Dark Elves then.
two alternate ideas to think about anyway.
Lawden only says Thor needs Loki to access the dark side of Asgard for reinforcements, he doesn't say anything about them bringing Jane along because she needs the possessing spirit exorcised, so again, it may be something that's not noticeable (like the way Loki watched over Erik at the end of Thor 1, not necessarily controlling her completely just maybe Malekith using her to spy on things, without her knowing it)... and so it may not be any bigger a piece of the story than that.
there was never any confirmation that Natalie was in Iceland (and I think she was confirmed to be elsewhere) but I don't think that necessarily means Jane won't be there, just the scenes didn't require her there. Although I'm still not crazy about her tagging along.