I have read so far what has happend and just want to give some of my advice. Ive had several Shih Tzu dogs in my life but there is one I had I can relate to you with.
We had adopted a 4 year old male Shih Tzu from the pound once. He had been in MANY homes and always brought back for the same reasons.
Not potty trained, and not easy to handel.
How we found him is, we was there one day just looking for a dog. Then in the back, in small cages in a "restricted area" I seen him. He was nothing but a ball of knots, and his own feeses (sorry brain cant spell). I asked about him and they told me he was going to be put down! I asked why and they told about his problems and hes always being brought back.
Well after much begging and talking, I was able to adopt him. I wanted to see him in a good home, I felt so bad for the little guy. A few days later, I was able to bring him home. Now while at the pound, oh he was so happy to get human attention. To be out of the cage and moving around! I was able to hold him in a towel (they had not cleaned him) untill we got to petsmart. Were had him bathed, hair cut and checked over.
When we got him home.. his whole attitude changed. Same with the problem your having, except we didnt have a crate. He went under our couch and would not come out. If you even SAT on the couch he growled and snaped at any movment around it. After a few days of trying, we thought you know this is not going to work he would not eat or anything. Finally he did start eating.. he came out at night and ate and drink (going to the potty under the couch was not a pleasent smell) Of course he had it on him also and like yours, finally came to us like telling us he wanted to be cleaned. Shih Tzu are very clean dogs, they love to be groomed and washed and pampered so its not surprise she loved it when you did.
It went good for a few days but he would at times go back to bitting, growling from under the couch. It toke us about a good month or more.. before he stoped this behavior. It seemed we could not force ourselves or anything onto him, he picked when he wanted contact with us or anything eles.
Really giving her back is not a good idea at all, since these dogs get very attached to there owners and certain people. You say she did ok with your son on the ride home? and great with your husband? It sounds to me the kind of people who have abused her were females.. is why she is not accepting you right now. our Shih Tzu , did not like males. at all. He would go out of his way just to let any male know in sight how he felt. Really just try sitting in the same room with her. No contact, no talking just sit near her (not in biting reach of course) to show her your not going to harm her or force her to do anything. Also dont keep the crate, or try to keep her in it. Shes from a puppymeal, that is the worst thing you can do for this kid of dog since she was rasied so badly in one.
Really I hope things go well for you, for Shih Tzu are the most loving dogs in my opinon. They really can act like people, with attitude and all.