Boo hoo on those blues. Are you giving Errol a snack at +8 or +9 during the day? That might bring down his night time numbers.
Cats do take in more water with a wet food diet. Is Errol still drinking out of his water bowl too?
For hairballs, there are a couple of options. Some people use a bit of unsalted butter (which mine LOVE!), or organic virgin coconut oil (also well received here), and I've also seen people give olive oil or fish oil. If he is working on a hairball, that might be why he's not feeling so hungry.