Finally finished my second playthrough with a full party. Baardwark's advice worked wonders
... apparently raising an NPC's attitude to 100 "resets" the counter somehow and you can "poison" your companions a few times again. Some sort of a workaround until this bug gets fixed (if ever).
Then again: I experienced the companion stories for the first time. Strangely enough, they don't raise their attitude towards you after completing their quests. Funny they should be so sensitive about friendly fire in the first place.
