If it's from Tobi then it's a loss of hair cells in the ear. Currently there is no way to reverse this. Magnesium I believe is a good preventative for noise-induced hearing loss, not medication. I believe that studies have shown that low magnesium levels can hinder hearing with gentamycin.
As for the hearing devices in the theater most just increase the volume of the movie. Basically it's like sitting in front of the TV and turning up the volume all the way. If she's only had high frequency loss then she'll notice it, at least the lower frequency which might be why she's so excited since she's hearing something. There's no guarantee that her hearing will get worse, but then again with Tobra there's no guarantee that it'll stay the same either. After having my first 'hit' of hearing loss it didn't change for about 2 years and probably wouldn't have changed if I hadn't gotten so sick and been put on a variety of aminoglycoside medications.
As for the insurance information there's not much I can help you with besides saying look into your policy. A lot of policies say that they will provide hearing aids as long as the person is under 21. If she's on Medicaid it depends on the state; with the state that i'm in right now PA I'm eligible for hearing aids up until 21 but it varies. I also have secondary insurance and I'm eligible for hearing aids through them for a small co-pay depending on the brand. So it just depends, you won't really know until you do some in depth research about it but not all insurance companies deny hearing aids right off the bat.