Tenebrium weapons and the weapons you find if they are the same level can have the exact same base top damage. Theorethically this makes them equal, sadly enemies in the game seem to have a very low resistance to tenebrium compared to all other resistance types. Thus you will do more damage having a tenebrium weapon due the less of your damage being absorbed.
Therefore my conclusion is that a Tenebrium weapon is better, however it is not so much better that non-tenebrium weapons are useless. However there is a part in the game where you are required to have a tenebrium weapon to accomplish a certain task. This ofcourse does not mean that you need to go for tenebrium weapons or that it makes them better then non-tenebrium weapons, however just saying that somewhere along the line in the game you are required to have a weapon that deals tenebrium damage.
Once you add any form of tenebrium damage to a weapon it will use it's tenebrium skill regardless of the normal weapon skill it used. (You already noticed this and you are indeed correct.)
Adding elemental bonus to weapons depend on your crafting skills and as far as I know you need crafting 2 for a small bonus and crafting 5 for a medium bonus, the exception being joshua's spice which gives a large fire bonus. However you can easily test this yourself by just playing around with crafting, save before experimenting and then reload. Ofcourse the higher level weapon you have the higher bonus damage will become.
I don't really understand your last question, but I think you are asking if tenebrium skill infulences tenebrium added elemental damage. If that is the case, then no and yes. That's a really great answer isn't it? :P
I shall try to explain it.
You already noticed the moment you enchanted a weapon with tenebrium it will use your tenibrium ability, so once you add tenebrium to a normal weapon it is no longer a normal weapon. Your tenebrium ability affects the bonus % damage you gain when using a tenebrium weapon. So no it does not directly affects the amount of damage you deal with added elemental bonusus. And yes it will directly influence your damage if you use a weapon that counts as a tenebrium weapon.
I hope that was clear enough.
With kind regards,
Rashar.
