Here are the important things to keep in mind:
The shots need to be 12 hours apart, and as close to the same times as possible. Consistency matters. Prozinc has some flexibility in terms of being
safe, but works best when it's consistent. So shooting as close to 6:30 each cycle as you can.
The tests also need to be consistent, and as close to shot time as possible. In the last hour between +11 and +12, many cats, especially cats in higher numbers, can rise over 100 points. So if you test at 5:45 one day, and 6:15 another day, and 5:30 another day, your data is going to be kind of messed up. It also messes up your mid-cycles since +4 might actually be +4.5 or +3.75 if the test/shot times were varied.
Now as for the feeding: it's totally fine to feed multiple times per day. You already know about the 2 hour fast before shot times, so as long as you're keeping to that, the feeding times aren't that important, although they do influence the curve, so again, consistency is important. It doesn't matter so much when or how many times you feed, as long as it's as close to the same every day as you can make it. Whenever a cat (or human) eats, the body releases chemicals and hormones and keeping those consistent helps with everything else in this process. It's less critical than the tests and shot times, but still very helpful in getting the cat regulated.
As far as eating right at shot time, the reason for that "rule" is to make sure that the kitty is willing to eat that day in case they start to drop low and you
need them to eat something. Sometimes a kitty will stop eating for whatever reason (infection, ketones, pancreatitis, etc.), and no food + insulin is bad. However, if the kitty is eating that day, but maybe not right at that minute, it's usually okay. If you can even get him to eat a tablespoon of food at shot time, it's a good little guarantee that he's okay. My cat grazes, so he doesn't usually eat a meal at shot time. He just eats a bit of his food and then I shoot and he goes and takes a nap.
It sounds like maybe what' happening is that your kitties are hungry before shot time? It can take some training to get them onto the needed food schedule, but it is possible to get them adjusted to waiting a bit until it's time. Just slowly keep pushing back their dinner time until it's closer to when you need it for the test/shot time. There may be some vocal complaining for a while as you make the shift, but they really will be okay. I sometimes had to go for a walk during the 30 minutes before dinner time because I'm such a softie it was hard to listen to them complain and not give in!