Does the vampire skill Poison Talons work when you’re not in vampire lord form

It just adds 50 points of armor to your armor rating. So if you are already capped out, it doesn't do anything (other than the extra magic resistance).

This can be seen by checking your armor rating before and after getting the stone.

You can recove your soul after accomplishing your objective in the Soul Cairn, Valerica will give you the quest, it is fairly trivial.

Should you become a vampire, you will need to be cured in order to continue working with the Dawnguard, you'll get the quest to get cured from members of the Dawnguard.

Interestingly enough, getting your soul back will net you 1 black soul gem, while getting cured of vampirism will cost you 1 full black soul gem.