Here's the bottom line:
Despite the fact that this would have been the third year he's been on it, Paddy's winter feed affected him metabolically in some way. We switched him back to his regular feed (Triple Crown Lite), and within 36 hours his personality was totally back to normal. The vet even joked about how sneaky he was being (he was trying to flip over a table of equipment and was mooching the vet tech for treats). So, no more Triple Crown Senior for The Padster. I need to find an alternative way of getting more calories into a hard-working Haffie - right now I'm looking at ground flax and maybe some soaked and rinsed beet pulp, or possibly a touch of alfalfa. I just have to keep it super low NSCs, because our guess is that was the trigger. Given the fact that he was his normal self, the vet said that Lyme or PSSM was highly unlikely, so we did not draw blood to test for either of those. However, if symptoms should continue, we'll definitely do that.
Nope, I'm not trying to escape. I mean, you left the gate open - what did you EXPECT me to do???
That crack I mentioned in the RF heel? It's MUCH deeper than we originally thought. I started digging around Tuesday night and basically sank 3/4 of an inch of hoof pick into a thrush-filled cavity. EEEK! I used a syringe (with no needle) to flush out the cavity with Betadine solution, jammed some anti-fungal goo in there, and packed an entire piece of gauze up in his frog (this both keeps the goo in and the dirt out, plus it helps get some air up in there - and thrush doesn't like oxygen). Let me tell you, it's disturbing to be able to pack a whole square of gauze in your horse's frog. When the vet came out, Paddy was positive to hoof testers on the RF heel, and was slightly short on that foot as well. There was no heat or swelling anywhere else in the limb, and he had no back pain, but his shoulder and neck were very stiff - as one might expect with a sore heel. So, we're now on a 2x/day flushing and packing routine, and even after just a day, it's looking so much better and he's moving more freely. The plan is to continue this for a week and hope it clears up and he goes back to being 100%, and if not, we'll reassess then.
Thanks to everyone who commented with their experiences! The combined knowledge of bloggers is pretty cool. I have to say, I'm glad it's not PSSM or Lyme, even though the sudden personality change certainly made those two options seem likely. I've had plenty of experience with metabolic issues with Saga, but never seen a personality change quite like that. Oh well, each horse is an individual - live and learn!