You do continue increasing following the protocol as the increased glucose can cause other problems.
By following the protocol, you safely increase, attempting to match the glucose levels with the insulin. If it is pancreatitis, treating that may help.
As far as the vets recommending further tests. You can do them if you want to be as accurate and certain as possible of the diagnosis. Or, you can discuss empirical therapy with the vet(s), where you treat "as if" that is the diagnosis. The problem with empirical therapy is that if you're wrong, the treatment may not help and could harm.
That said, B vitamins are water soluble and not likely to cause harm at a therapeutic dose.
Subcutaneous fluids may put a water load on the heart and kidneys; amount must be regulated.
Buprenorpine for pain may make the cat groggy and in some cats, a bit aggitated.
Digestive enzymes, which may be a result of pancreatic damage from chronic pancreatitis, are available over the counter (ex Mercola makes a product).
Allegedly, fat content in the diet for cats doesn't matter; some cats may do better with less fat (just add plain poultry to the canned food.)