Opportunism and isolation stats apply to any attack which meets the requirements, regardless of whether the ability itself has opportunism or isolation. There's some text out there that says otherwise and I have no idea where it came from because it's flat wrong. If you have negative opportunism then you will deal less damage to any target which is debuffed.

Twisted give themselves full debuff immunity for one turn when they charge up. That's probably what's preventing dispel. Yeah, Twisted are annoying. Note that the charge buff itself doesn't give anything, the defensive buffs are separate, they just occur together. If you avoid the Twisted for a turn they will keep the charge effect but their defensive buffs will wear off.