Hmm hmm, as far as I know, you don't apply spell sustaining modifiers to any damage resistance test, you don't apply confusion, or anything else. If modifiers apply to a damage resistance test, whether it is a drain, or technodrain, or a bullet you take in the chest, or anything else, if a specific modifier applies, it'll be said very precisely, probably. But being confused doesn't make a troll less resistant. The confusion spell modifiers will already apply to the troll's defensive test.
The modifiers you apply generally depend on the damage source (APDS ammo, Ex-ex, classic drain, poison or toxin), and your defensive options (armor, cyberware against toxins, racial modifiers, and so).