I know we sound like broken records about mid cycle testing but quite frankly, without it, it's impossible to know what is going on. It's quite possible that Fluff is dropping a lot lower than you are aware of on some occasions and then bouncing up high again and those highs can last for 3 days. This leads to a perpetual roller coaster and what can look like persistent hyperglycemia.
When we speak about mid cycle testing, that test can be at any time during the cycle. ANY mid cycle testing will help but you have a lot of days where no testing is done other than pre-shots and that doesn't tell you much other than it's seemingly safe to give insulin. The question is not just whether to give insulin but also how low the dose is going to drive BG.
How is Fluff's feeling overall? Is he purring, playing, preening, peeing, pooping? Has his urine output changed at all from when he was diagnosed? Does he hang out by his water bowl? How is his appetite? When is the last time you had the vet check his teeth as that often causes elevated BG.
Given that your vet started out at 1u but then increased in full units at the beginning, I have to still wonder if the dose is way too high. I would expect that we'd have seen some low numbers at some point in time if that were the case but without more mid cycle testing and all the unknowns that leaves, it's still something to consider. I do find it interesting that the pre-shot following skipped shots always seems to be reduced rather than elevated as would be expected.
With those persistently high BGs I'd suggest testing for ketones regularly and reduce the dose back to 4u and see if numbers improve. Right now it looks to me like the numbers were better somewhat better around the 2u dose.
It's possible Fluff could have a high dose condition or that the ProZinc just isn't the right insulin for him but without more mid cycle tests to truly see what is happening, all we can do is guess.
I'm tagging
@FurBabiesMama to ask for another opinion.