Hi and WELCOME to YT...glad you found us
. So sorry your kiddo is going through this. I don't have experience with shunt so I won't speak to that. But, I do want to ask if you've tried other foods bc oftentimes when there is gas/indigestion/bloating...it is bc there is something in their food that their system cannot tolerate, same as in humans. I would be trying a different food. I'd also ask the vet for something for the tummy for if/when it's at its worst.
As far as the legs giving out, does this *only* happen when you're seeing these low blood sugar symptoms? If not, then I'd be concerned about her back/discs...and she should get an xray of her back and neck if they give out at times other than hypoglycemia.