There is a book that can be found in Sacred Stone to learn the Tenebrium ability, and Brandon will accept any tenebrium ore, not just that mined in the troll cave. Assuming a party isn't dual Lone Wolf characters, Bairdotr starts off with a level of Tenebrium, and if you use a henchman temporarily, it wouldn't matter too much if they got rot (their HP would eventually drop to 1 if not cured by a star/blood stone, but they'd be fine hanging out in the homestead like that).