First of all, congrats on having a diet controlled diabetic kitty!
You have proven that the food suggested was wrong; dry is bad. I know for a fact that it raises BG from Shadoe stealing just a mouthful from my civvie. Her numbers soar to the 400s. Stick to the wet, and ff is fine, but you can check for others in Binky's list. The list is also good for other values than just the carbs, so if you are looking to shave a few lbs off...
Binky’s Food Lists
Feeding Your Cat: Know The Basics of Feline Nutrition
The insulin you were giving apparently is very old school and harsh; if you are to find in the future that you need to start giving insulin again, you would do much better with one of the more longer lasting and gentler insulins like Lantus Levemir or PZI.
I have tried Proplan for mine and they ate it, but prefer their friskies and some of the ff. How about if you get just a couple cans to try and see what flavors he likes.
As for not telling the vet yet? Excuse me but whose cat are we talking about here, and whose action got him off insulin?
The vet's directions? NOPE.
Your actions? YESSIREE!
You can tell the vet that the vet's instructions were causing your cat to go hypo because your cat needed only a diet change. Vets make money off that food they push on their clients, but they need to become more educated I think.
The food you are feeding could be split up into smaller feedings if you like because you want to keep the BG smooth during the day so more frequent eating will help there. You could us an auto feeder, or if you are home, just split the 2 cans into 4 or more meals. Most people don't eat just twice a day so cats should be treated the same I think.
Petsafe 5-meal Auto Feeder
Bottom line: you have done a fine job for your kitty. He's off insulin and doing just fine ... thanks mostly to you