Hi. I find it very odd that the vet told you to give a different dose for AM and PM. Normally, the dosing should be consistent. Based on the numbers you have, the insulin is basically having no impact. Bryant's dental issue is very likely keeping his numbers high. Pain and infection will do that. So, it sounds like a catch-22 that the vet does not want to address the dental until the diabetes is under control. What exactly is he waiting to see in regards to BG numbers before doing the procedure? Mia had dental extractions while her numbers were staying in the mid-200s up to low 400s. She was definitely not regulated at the time, but her numbers were not as high as Bryant's. (She also did not have an abscess to drive them up that far.) She took Clavamox starting a few days before and lasting until a few days after.
Do you have a veterinary specialist clinic in your area? If you do (and cost is not an issue), you may want to see a dental specialist for the procedure. I have taken both of my girls to one (board-certified veterinary dentist; Diplomate of the American Veterinary Dental College) for their extractions, and I will always go to him from now on for any dental anything for them. It is just a different level of care than you get from a general vet, even the anesthetic protocols and monitoring that they do. (Mia also sees the internal medicine specialist at the same clinic for her diabetes, and it made me more comfortable to know they were talking/coordinating.)
That said, I stand by my comment about needing significant time to get BG under control.
