Nav was lame the next time I went and saw him (last Thursday), so we decided it was time for another trip to the vet. We went to a different one this time.
He found a very soft and sore spot on the right front, near a deep separation. He had Nav lunged and he was clearly off going both directions, but definitely on the right front, so he blocked the nerves to see if the lameness was anywhere up the leg. Thankfully, after blocking his hoof he lunged "sound". Vet determined that the separation is a pretty deep white line disease, and that's what we are treating for now. I'm currently packing his hoof once a day with cotton and saturating it with a solution. If this does what it's supposed to, he should start to heal up and we'll just need to keep the separation very clean as the hoof grows out. But if it doesn't clear up with the current treatment we'll likely have to resect that hoof, which will be a huge ordeal and much more to take care of. I'm praying that this will clear up with the packing and solution application.
Nav can go out in the pasture with the packed hoof, the crack is narrow so the cotton doesn't dislodge easily, but for now he's being stalled just to keep him from moving too much - he was super lame the day after the vet visit - and to keep it dry, since we've had rainy days lately.
This is all quite frustrating, I usually am very careful about clearing and cleaning separations so that they don't get this deep, so I feel like it's a bit my fault. But, I'm glad it's nothing like a joint problem, and I am glad we have an answer this time.