Hello and welcome to Paw-Talk!
I've had similar problems with my Doberman as far as the intense curiosity with cat butts. It seems I'm constantly telling her NO! I even went as far as borrowing my friends shock collar and zapping her when she got close to the cats. It worked for awhile but then she got smart and realized it was the collar zapping her and not the cats......so every time the collar was off she was after the cats.

I'm back to yelling.
My dobe also had the separation anxiety right from the first day I had her. At first it was absolutely horrendous. I would walk out the door and she would howl like someone was trying to kill her, then she would go shred whatever she could find. She would even have a fit if I went into the bathroom and shut the door. I got her a HUGE crate, put alot of toys in it and would put her in there and give her a treat everytime I left. I started off just walking out the door, then coming back in after a few minutes to show her I WAS coming back (something to do on your days off from work). She still gets nervous when I leave, but she doesn't howl and carry on like she used to. I have noticed she is much happier when I put her outside for the day in the summer time. I have a 20x30 kennel and she loves it outside in the summer.
As far as the leg licking thing, I think that's a nervous habit. Mine used to like to bite the end of her tail and make it bleed. Once she got over the majority of her separation anxiety, the tail biting seemed to stop. You can try putting Vicks Vapor Rub on it to try to deter him.
I really don't know what to suggest about the drooling problem. I know quite a few dogs that do this and they have to be constantly reminded to stay away from the kitty box.
I'm not sure if neutering will help his problems. My dog has been spade since she was 6 months old.