Jesus. That's quite a lot of information.
I was frankly thinking about making a Dark Troll out of some rider model (say the Far Seer) and mixing her up with a modified Spider model (definitely thin legs, similar to an older test-model I made; probably some voodoo masks and armor on its body somewhere, still not sure what to do with the head - either make a new one or edit a Nerubian one, I suppose) with Retera's Matrix Eater. Making him male kind of defeats the point in my opinion, but I can work with that, I suppose. My idea was just to swap from a purple texture to a blue one to suit your needs; In my eyes, spider-witches and such fit the Dark Trolls better than they fit the Jungle Trolls.
My main issue with re-modelling the spider is, frankly, my arachnophobia. That makes looking at spider anatomy images and such very unnerving for me. Thusly, I cannot promise anything particularly spectacular, but I agree that using the default spider model \ head would be very lame. This concept in particular is rather inspirational - however, in that case, in order to retain unit proportions and not go full hero mode, the troll would have to be a tad less detailed than I would have liked.
...Which is okay, though, because this concept is cool, and fixes quite a few problems. The troll will be less detailed, but he could have those fancy spider-limbs stuck to its back in some way to make up for it. It might be too detailed, but we'll see. I'll have to find more ways to make the troll "spider-like". Fake spider-eye tattoos? Clothing made out of web? Different looking fangs\tusks, dripping with venom?
Keep in mind this is all in conceptual stage, and while I am rather excited thinking about it, I cannot promise I'll ever actually execute any of the above.
P.S: I'd rather not take any parts of -Grendel's fantastic Conjurer model. It's not really my style to take things from other people, considering I also want to make this model for the sake of modelling stuff, your project being a convenient creative outlet which also makes me feel like I'm helping someone.