Bless you for rescuing this Angel. He sounds typical Yorkie, lol. They were ratters originally so going into dark places is normal I would say.
His personality will continue to develop as he gets more comfortable with his people.
Potty training is a consistent thing, take him outside to the area you want him to use and when he goes he should get Lots of praise. Treats for this action is up to you but if you start of with treats you have to wean off of them Very Very slowly OR they think they are getting cheated, lol. They are Smart little dogs.
Congratulations and welcome to the world of Yorkies.
Just a thought........we LOVE pictures!!!!!! I have not managed those yet.