I agree with Joanne that you really need to get a behaviourist in to observe her and your interactions with her. It sounds like she could be developing some resource guarding issues, and continuing to confront her over this will only make things worse...
I don't understand why you say you can get on her bed and you can take her food...
This really isn't the way to gain her trust, and if her bed isn't somewhere safe for her this is probably why she has adopted the area under the bush as her den.
Rather than taking her food off her (which will only serve to make her more worried that you are going to take everything else off her too) try adding tasty additions to her food... if necessary start by gently throwing them towards her from a distance... she needs to learn to associate you (and your family) as a provider rather than a taker-away...
As far as the balls are concerned - just don't get into a battle with her - if 2 balls still create conflict, take 3... show absolutely no interest in the one she has and work to get her interested in the one you have... don't call her and show her the ball, but play with it yourself, try to hide it from her view, act like you are really enjoying the ball you have... she will soon lose interest in the boring one she has and want your really fun and funky one!!! (Believe me, we have been going through exactly the same issue with the balls... I can now get her to retreive a ball to hand when necessary, but we still use 2 balls for regular training sessions on balls, so that she is continuing to learn to reduce her focus on "her" ball and switch her focus to what I can give her!!).
Oh, and she was soooo obsessed to start with I needed about 8 balls in the garden to begin with, as she collected them all up into a pile!!
Another thing we were advised for the balls was, rather than to throw the ball away, to actually get her to come up and take it from us, or to throw it to her to catch... this stops them seeing a ball as something you take away from them (we know we take it away to throw it for them, but they don't always make that connection!) and makes it something you give to them!
Good luck!!!