1) Pickpocket seems to be one of the most valuable skills for a pure thief. With misdirection and perfect touch, you can render your enemies virtually unable to do much damage to you. Furthermore, with the perk poisoned, you can kill your item-less target by putting poisons into their inventory. Of course, this does require you to be a very successful pickpocket. With the use of alchemy, this can, I assume, become incredibly easy to use to kill enemies without ever touching them. From my understanding, even if an opponent has poison resistance, unless it is 100%, the poisons will still do damage, you will just need a lot of them.
2) Sneak is also key, as one can assume. If you're caught, it is almost game over, as you have no combat skills, except for poisoning your enemies. However, using this for the damage bonuses is not the goal-no weapons.
My only concern is how to handle bosses, and dragons. Off the top of my head, I don't remember if you can sneak around bosses. I'm fairly certain you can, but it has been months since I had to. I don't really intend to fight dragons, unless I find a follower to do it for me.
Lastly, are there enemies I would be facing that have 100% poison resistance? I, honestly, have never used poisons all that much, so I'm not really that familiar with them.