Argggh. Your vet did exactly what I was afraid they were doing, which was use the wrong insulin, not understand how the cat's pancreas works, and then seriously overdose him. Giving 6u of Humulin N in the first week of treatment is
insane, and you're lucky your vet didn't kill your cat. I would NOT go back to that vet or take any advice from them for
anything concerning his diabetes. If you want, you can try and educate them if they are willing, but with someone that far behind you would be doing all the teaching.
When a cat drops into low blood sugar, their pancreas will release a hormone called glucagon--this is like the opposite of insulin. It tells the liver to release glucose into their blood stream to counteract the low blood sugar. So the more a cat is overdosed and dropping into low numbers, the more their numbers will bounce back up and hit very high numbers. This keeps happening until eventually the pancreas can no longer keep up with the low blood sugar, and you have a severe and potentially deadly hypo incident. That your vet does not know this and kept increasing the dose
despite seeing very low numbers is simply unacceptable and very dangerous, in my opinion. You certainly did the right thing NOT giving him insulin once you got home. considering that those very low numbers at the vet were stress inflated already, giving a dose like that at home once your cat relaxed and his blood sugar had lowered some would not have ended well.
Here's the dosing protocol for Humulin N:
http://www.uq.edu.au/ccah/docs/diabetesinfo/link2.pdf. Note where it says "Lente [Vetsulin] is the 3rd insulin of choice and NPH [Humulin N] the 4th of choice insulin for control of diabetes mellitus in cats, behind glargine [Lantus] or detemir [Levemir] (1st choice) and PZI (2nd choice), Lente and NPH result in lower remission rates compared to longer acting insulins." Also note that it states the dose is not to exceed 1u for the first week, and not raised by more than 1u every two weeks.
However, I would get a different insulin ASAP. Humulin N has a very low remission rate and is pretty ineffective in cats because of it's poor duration of action with cats' faster metabolisms. Call around to other vets in your area and ask if they prescribe Lantus, and/or say you have a diabetic cat that's already been diagnosed by another vet and you want to switch to Lantus. You should be able to find someone who'll be able to get you the script.
Here's an article that you can also bring to your or a new vet to get the prescription, if you encounter some resistance.
Here's a link to the dosing protocol for Lantus that's outlined in the article, which has the very high remission rate:
http://www.uq.edu.au/ccah/docs/diabetesinfo/link4.pdf. This is very useful information to have because some vets can be great in most aspects of treatment, and then not know how to correctly dose the insulin in cats (many want to raise the dose too much to quickly, albeit not at the same insane rate your current vet did).
Sorry if this post sounds a little rant-ish. Most vets don't know every single detail of diabetes treatment with cats, so it's very common for diabetic cat owners to stray from their vet's advice once they've educated themselves. Vets are general practitioners that must know many general things about many different diseases for many different animals, so it's unrealistic to expect every vet to get feline diabetes treatment perfect. As long as the vet is fine with you taking charge of your own cat's treatment a bit (and many vets welcome and encourage this), then getting something wrong is not necessarily a reason to leave your vet. However, given the dangerous nature of your vet's treatment, I would not only leave, I would make them very aware of exactly why I was leaving, because it might just save the life of the next diabetic cat that shows up at their office. I'd also put up a huge stink about the bill for keeping your cat and subjecting it to their "treatment" for
eight entire days. If you've paid the bill already, I would demand at least a partial refund--what they did is ridiculous.