You've diagnosed things pretty well. Your dog is jealous and territorial.
Sounds like there is also some fear aggression going on - moving from hiding and barking to biting is not good. Dackels are infamous biters, and they really need good training with an owner that is comfortable being the alpha in the pack.
Has your dog had any obedience training at all? (I don't want to start at too basic a level if you've already done some work with the dog)
You need to work on improving your alpha position as well as increasing the dog's confidence. I love Dackels (esp. the long-haired standards) but they do need a firm pack leader.