When Tucker crashed his potassium got as low as 2.3, we were never able to get it above 3.2 - 3.4, we tried, but it just didn't happen. He took the pill form, we crushed it and put it in Soulistic canned gravy food, the tuna kind from Petco. It's gluten free so it was good for his IBD and didn't raise his BGs. But Tucker was different from other cats, he also started prednisolone for a brain tumor at the same time and his BGs still went down. I believe for him the pain relief, pred making his tumor shrink equaled less pain, therefore lower BGs.
CRF kitties, hypert and even FDs can become hypokalemic due to the amount of water intake and output.
Perhaps you could talk to the vet about adding the K straight to the fluids. Since you're giving subqs anyway, why add more stress with another pill? When Tucker needed fluids for pancreatitis flares, our vet added K to the bag but he took that in addition to his pills because his hypokalemia was so bad.