I got great news today! The farrier came out and did his own evaluation and whatnot, and I was very pleased with the results.
First, while he isn't saying Flash didn't have laminitis, he determined that the primary cause of the lameness is thin soles. He's got about 4mm of sole when he should have nearly 3x as much. In fact, when I was cleaning out his feet beforehand, some sole flaked off. When the farrier came out, he showed me the same indent I had noticed earlier and showed me how, when pressed there, Flash reacted. For once, I was happy I could easily see what the farrier/vet was talking about. It was funny because when I said I saw it, he asked "do you really see it or are you just saying that you do?" so I was glad that I actually did see it lol
He's got the same thing going on in both fronts, but no sole issues in the back, which is normal. He just put regular shoes on the front to get his soles off the ground. I need to put cotton in the space between the shoe and where the soles are (the shoes cover it up a little, so it'll hold wedged-in cotton well) and soak it with iodine to get what sole he does have to harden up. I need to do that every 2-3 days for the next three weeks. Apparently it's a genetic thing with a small percentage of horses, so I just happened to get one that has that problem. Once we get down to Vegas and his feet have hardened up, I should be able to do a re-evaluation and possibly pull the shoes. To keep his feet healthy, he needs a hard, dry environment and plenty of exercise to keep his soles stimulated so they grow plenty of sole.
With proper care addressing both the risk factors for laminitis and encouraging him to produce plenty of sole, he should be just fine and even go well barefoot.
He had me walk him around with the shoes on gravel, including quick turns both directions, and he was moving well and stepping over himself, so he says there's no founder at all going on (though he could have had a brief but managed laminitis bout).
For the record, Flash was absolutely wonderful during the entire process and didn't show any signs of pain when the shoes went on. In fact, he was licking my hand most of the time - I must have had some Ultramin or something on it from making his mash. He was such a good boy and seemed quite happy - I was impressed with how well he behaved because he's usually not even that good for the barefoot trimmer!