When this insulin question first arose last year I think, the alternative was an insullin used by humans.
It does seem that quite a few cats do not do well on the one that Harry is on and I think that Ruths Lexy was one, and she is now in the States.
I do remember Lexy became very ill because the insullin was not working properly and once Lexy had been stabalized the insullin was changed.
I think you need to go to war on this cos this vet is dodging the issue by all accounts and Harry needs to be stabalized asap.
Sending you some boxing gloves and loads of luck in sorting the vet out.
Oh another thing, as long as the medication whatever it is works well on cats, with the owners permission unlicensed products that are used on humans or other animals can be used on cats.
This goes back a very long time because some human eye drops were used on my cat Kocka 10-15 yrs ago, they were very expensive and had to be got by the vets from Moorfields Eye Hospital in London. Metacam is another example because the cat version is relatively new but the dog version I think was being used on cats.
Because we are getting private medical treatment for our cats, so many options are there if the vet knows what they are doing.