Hey, a little bit of his regular food sounds like it works fine as a treat. Non-food rewards or treat ideas also sometimes work. Ear or chin scritchies, brushing, petting, talking to him and telling him "What a good boy you are Hairy! Such a good boy".
From your description of Hairy's behavior, it does sound like there has been some overall improvement. Yeah! :RAHCAT
It's unlikely that the insulin has gone bad already. Some people keep it 4-6 months in the fridge. Sounds ok from your description.
It's difficult to tell what is going on with Hairy. Some possibilities are;
1. the dose was increased too quickly, in too large increments and you bypassed a good dose.
2. you haven't found the correct dose for Hairy
3. you could have given a fur shot. Have you been running your hand through the fur to check for a wet spot? Or smelled the insulin, like a bandaid smell after shooting?
4. Hairy could be bouncing off some low numbers. bounces can last up to 72 hours. The body panics seeing lower numbers than it is used to, and pumps out counter regulatory hormones and some glycogen to raise the numbers back to what it considers normal. It can overcompensate when doing this, hence the bounce.
5. Any access to higher carb food for Hairy? Another animals?
It's very common for cats to drop lower overnight. Is there any way you would consider getting a mid-cycle test at night sometime?
Cats can be resistant to insulin. More common in male cats and overweight cats, per this article.
http://www.uq.edu.au/ccah/index.html?page=43391&pid=0 Cats can develop insulin resistance with inconsistent dosing and other factors, I don't know them all. Cats can also develop IAA or have agromegaly, but we do not usually consider that until you are at 5U or more of Lantus or any insulin.