Poor little girl...I'm sorry to hear about this.
I know there are frequent issues with abscessed teeth in these little guys. We've been blessed not to have any, therefore I don't have much to offer, but I'm sure other's will.
I would think that if the root/nerve is exposed and causing pain something more will have to be done. If when the recheck her and it seems to be grown over, I guess there is the possibility that she may be able to live with this.
The sudden weight loss isn't a good thing, not for any living creature...however she should be stable enough at the current weight. The problem is that she seems to be continuing to lose.
I would seriously consider pulling her from her cage mates. It sounds like she doesn't stand a chance to get a good meal in with her complications. If she had the time to eat in the way that she needs it would seem that she could put on some weight and at least be getting her nourishment. Her nectar replacement staple shouldn't pose a problem though, as it is basically a thick liquid.
Hmmm. I'm interested to see what others have to offer and wish to keep tabs on how she is doing.
Best of luck to you both!