FWIW, and this is quite different from Legs' symptoms:
Several years ago my whippet Rickie suddenly started to scream any time he went up a step; nothing else bothered him; we saw several specialists, including, finally, a neurologist who had a lot of experience and who had owned Salukis. We did an MRI which showned nothing. She said she had once encountered something similar that turned out to be a ligament injury. So while she couldn't pinpoint Rickie's problem, she opted to recommend a plan based on symptoms - similar to your vet - leash walks, no stairs, give it time. It's long enough ago that I can't recall how long it took, but indeed the problem cleared up and has not returned.
is there any sign of flinching anywhere when Legs air-snaps? I had a hound do this when she had some tight muscles - you could induce it if you ran your hands down her in a certain way - massage and extremely gentle chiropractic cleared it up.
Again, this may be tangential to what you are encountering with Legs.
We have all fingers and paws crossed here that someone will find the answer, and it will be simple.