Yes, this is a very difficult and frustrating disease to treat when you have a cat that doesn't follow the rules. You could try feeding a slightly higher carb food to see if that helps. It's true that diabetic cats who are substantially underweight can be more erratic in their responses to insulin. You would dose around any effect the higher carb food has on BG levels.
Have you tested her urine ketones regularly? That's important because she's been in higher numbers for a while.
It's a small gain but her SS shows far more numbers in the 300s and 400s and almost no 500s or 600s as you've raised the dose. You had some hopeful action around the 2.5 u dose level but then she became stuck. Aside from her other health issues, she's likely struggling with glucose toxicity. It's a common problem and you have to be more aggressive with your frequency of dose increases - no more than 2 days (4 cycles) at a dose for now, something you've only begun recently. It's possible to raise the dose by 0.5 u for a while until you see some better numbers. That's not a large % increase and it might help to get her moving more quickly.
Many people worry when the insulin dose has to be raised beyond what they thought was "reasonable". Many unregulated cats are insulin resistant until a certain dose range is reached that seems to wake them up and they start to respond better. I don't think you're there with Goma yet.
On another note, given the difficulty you had getting a fresh vial of ProZinc last time, I suggest you get whatever process you need to activate going soon so you aren't caught with no insulin.