Hi Cherie,
I think you are right - the way your dog has behaved screams abuse at me, and of the worst kind. As you obviously know already you have an exceptional dog there. Put in the same situations many dogs would say enough is enough and defend themselves.
My own dog was pretty similar in background. Breaks your heart doesn't it?
Does she have her own bed away from things she can hide under?
Your aim has to be that although nothing bad happens to her while she is "invisible" she is just as safe while out in the open. You have to convince her that life is as good for her when she faces it.
If her bed has blankets or things she hides under remove them. A piece of thick foam covered in an envelope of sheet is ideal. Work on the command, gently and never as a punishment,"On your bed." Practice as many times a day as possible. In this case, maintain eye contact with her as she lies on her bed and after two seconds go to her and praise as if posessed! Encourage her to get up and play with you as part of the praise. Go mad!
Leave her to her own devices for five minutes, call her name in a happy tone and as soon as you have her attention say, "On your bed" as though it was the best idea you ever had! You may have to be sneaky and use food or anythng to get her attention but don't touch her and always sound like you are having a good time. Two second eye contact and praise again. Repeat this at five minute intervals until you see she is at ease with it. Repeat again, two second eye contact, two seconds looking away, go to her and praise madly. Always make your approach to her calm, only go mad once you have touched her.
Repeat again, two seconds eye contact and five seconds looking away and praise.
When she is happy stretch the non eye contact to seven, nine, eleven and thirteen seconds. When you have reached this pinnacle of achievement you can stretch the non eye contact times to up to a minute but as soon as she has achieved a long time without hiding go back to a two or three second test as a reward. This is a fun game for her and she won't realise she is being trained to not hide. As you build this up it is fine if she hides when you are not playing the game as long as you totally ignore her. When you want to go for a walk, someone rings on the doorbell, you are going out without her, you come home from work - anything that means people leaving or coming into the house - tell her,"On your bed." Don't leave, answer the door, put on her lead etc until she is on her bed. When you come in don't pet her until she is on her bed lying down - as soon as this happens go MAD!!
All that is happening is that your dog is learning not hiding is fun, nobody is anxious or upset if I do hide and I only get the attention and reassurance I need if I do things in a certain way, not hiding!