Since it looks like you got a broken cat and you don't want to return it, I suggest you call them and ask for a refund of the fees you paid. After all, they gave him to you broken and you shouldn't have to pay for it....that's my two cents.
Your thought process sounds logical to me - he was overweight, taken to the vet, tested there with a blood test, not a fructosamine - which would have given the average BG for the past couple weeks and based on one test alone, deemed diabetic. Yeh, you did a good thing rescuing him and getting him out of a potential dangerous situation. 3 units! Yikes!
Now you're sure you don't want to return the broken toy? ;-)