Bards being able to turn off trustall and hit anyone and everyone with their songs was a thing implemented sometime after Kastellyn was mortal, but also still long time ago.

For the very reasons that you have pointed out. In intracabal disputes, bards were rendered virtually helpless.

Bards can unquestionably turn trustall off and have their offensive songs affect cabal mates. Anyone who says differently is either talking out of their ass or has fought bards who didn't catch on to that.