Nice post Guldtasken, some interesting speculation.
Wanted to point out a bit here:
Unless it's getting changed (and we all know it could at any moment), the current lore indicates that the aqir became the qiraji, and then spawned the silithid. The silithid did not become the qiraji under the influence of C'Thun. In fairness this does contradict even earlier lore that hints at the silithid becoming the aqir/qiraji.The reason being that when C'thun broke through his prison to influence the land, he took contact with the ancient Aqir or well the remnants of it. Where he transformed the hive mind dumb Silithids into intellectual beings of Qiraji.
This doesn't really interfere with your theory vis a vis the trolls, just an FYI.
We really need that Warcraft lore book detailing the Old God's reign to come out and clear some of this stuff up.
Re. your other points, I'm only speculating here myself, but the trolls seem to have a strong connection to the world in its natural state - their loa spirits, their animals gods, their witch-doctors and druids and shaman, etc. Nature without the influence of the Old Gods, as well as the spirits born from nature and their own people, seem to be the focus of their worship and faith.
It's perhaps this mentality that makes the trolls resistant to the chaotic influence of the Old Gods. A sort've evolved natural resistance to it - while the faceless ones, the aqir, and the other races served the Old Gods, the trolls over time built up an immunity by worshiping other powers opposed to the Old Gods.
Obviously it's not perfect since there are troll worshipers of the Old Gods in the Twilight Cult, but perhaps the race overall has a greater resistance to them than others.