There is no dry food that is good for a diabetic cat. There are maybe one or two brands that have lower carbs, but then you still have the moisture problem with dry food. Also, even low carb dry food can increase blood glucose levels for many cats. It's best to just not have it at all.
What many of us do when we are not home to feed, is freeze portions of canned food and leave them out. If your cat's a gobbler like mine, you can get an auto-feeder to release the food at a specific time of day. If your cat is a grazer, you can just add some extra water and leave the canned food out like you do the dry. At room temperature the canned food is good to be left out up to 12 hrs, and up to 24hrs if it's frozen.
Please make sure you're decreasing the amount of insulin as you're changing the diet. Getting rid of the high carb food can cause blood glucose levels to drop significantly. Most cats on a low carb, canned diet do not need much more than 1u of insulin. Some even go into spontaneous remission.
Also, the dose should never be raised in whole units. Dose adjustments should be made in .25u-.5u increments, depending on how low he's going mid-cycle. 1u is a LOT of insulin for a cat. Bandit never needed more than 1.25u the entire time he was on insulin, and even .5u changes were far too much for him. He's been in remission almost 2 years now. Here is the Lantus dosing protocol, which is safe and effective (84% remission rate for the newly diagnosed) for cats:
http://www.uq.edu.au/ccah/docs/diabetesinfo/link4.pdf. It does require testing at least 3 times daily, which is very doable--you test before each shot to make sure it's safe to shoot insulin (this is recommended for everyone), and then you get a mid-cycle test to see how much the insulin is lowering his blood glucose levels.
As for the peeing thing--cats usually go about every 12 hrs but they can wait as long as 24 to go. A blockage is very dangerous and can prevent a cat from urinating, but there would be signs of distress, e.g. constant trips to the litter box, meowing in the box, straining to pee with no urine produced. As it got worse you would see lethargy, vomiting, and lack of appetite. If you see these symptoms, take him to the vet immediately!