Hi & welcome!!! From what I have heard, PZI Vet is good well past the expiration date. They don't manufacturer it anymore and supplies are running out, that's why you got one that's expired. I think everything that has been made may expire this fall, at least those are the latest dates I've seen mentioned. As far as I know it's not like it goes bad or anything past the expiration, if anything it would be that it's less effective, which you'll know sooner or later if you aren't getting a good response from the insulin. I'll be surprised if they can get you a new vial, as far as I've heard there's no where left to order it from. Maybe they will order ProZinc for you, that is the one that's more or less replacing PZI Vet.
Bix has stomach issues, and has done on the Wellness Chicken & Turkey flavors, which are 4% carb. The FFs (and everything else) made him sick. ECID (every cat is different) so you may want to try some things and see what works. If the food she is on works well for her you may need to stick with that though, and if it means she needs more insulin that may just be what you have to live with. Some cats don't tolerate the food changes well, and sometimes if it's working... don't touch it! I don't know the Royal Canin, but the prescription foods usually aren't really the best it turns out, so you may find she does great on something else. A lot of kitties have tummy issues, so there are threads you can search for on Health (or post a new one if you want) with people asking questions about what foods are best for stomach issues.