i'm sorry i can't help with the chewing
but i'm wondering, what does the area underneath her chin looks like? is it directly under her chin so the area where she can't actually get to with her teeth? which would suggest the fur fell out? is the skin smooth? are there any bumps?
quite often if there's an infection, there's a lack of hair. the reason i'm asking about the skin and bumps, my rescue girl, mae has a bone infection in her chin. she had absolutely no fur on her chin when i got her but her condition is chronic and it has caused the bone to extend so she pretty much has a tusk. she also has a sore which gets better and worse. this condition is treatable when it's acute. in mae's case it's too late as there are no antibiotics that could treat this that are safe to use on chinchillas. it doesn't bother her in any way, most of the time.
is she eating? has she lost weight?
i would suggest a trip to the vet. the stress can be caused by pain so maybe if she has pain relief she'll feel a little better?
good luck!