What would happen if your family offered to act as "foster parents" until she turned 18? This situation would make less work for CPS to do, and could be a temporary solution until your GF turns 18. Since it's only 5 months away, I don't see this as a terribly unreasonable option. If her parents are as nuts as you say they are, the toughest part of it would be finding enough time to gather her things and transport them out of the house, without the parents noticing. If she started packing her shit early in preparation for leaving, her parents would notice, so she'd have to try to cram it all into a day. I dunno, it's pretty hairy.
On the subject of the law, you guys need to remember that even though CPS and similar agencies side with the child initially, if the case goes to court the child still needs some sort of physical evidence to ultimately prove her case. Without this, no official charges can be brought to the parents, and the parents can get the court to give them back their child.
To be perfectly honest, I think what you and your girlfriend need to do most is to take a step back and plan to ride out the next 5 months until she can get out of there. As the sig. other, you need to be physically and emotionally supportive to her. She's undoubtedly going to run into more shit before she moves out, which you need to be there for her for.
However, I'm spacing this paragraph apart from the previous to put emphasis on it. You must NOT, under ANY circumstances, physically involve yourself in the situation. Ever. Though you may feel it is the right and noble thing to do at the time, you are just causing her damage in the long run. Your girlfriend should do her best to just avoid any potential points of conflict, and keep to herself for 5 months. Though this may limit the time you get to see her a bit, I think if you love her this should be well within your willpower to do. I went through (and am still going through) a situation with my fiance's father that demanded I maintain a similar position, so I know how it feels.
I know it's hard to stay out of it, but it is the best thing for you both. Just keep your eyes on the prize. 5 more months of Hell and then it'll all be over for good. No more controlling parents, no more stress, no more heartache. 5 months. Keep that in mind and I know you guys will make it.