Technically, Tenebrium isn't really that good anyway (especially when you prefer weapons without a gaudy elemental effect.)
Story-wise you shouldn't learn to handle it incidentally and within a few minutes from the very guy who could have prevented the immaculate's rise by teaching the same to his mining staff. A source ritual would have made more sense - to use it safely you'd need to sacrifice a litter of newborn kittens and bathe in their blood.
As for the rift, you don't need tenebrium to destroy the bloodstone itself, and you can sneak past the tigers or use invisibility, but well, the whole rift sequence (actually the whole Luculla forest) seems like hastily jointed filler content. As if this area was done and now needed a reason to exist, but there wasn't enough time/money left to do it as originally intended.
