I think the farrier digging out Tuffy's sole is what has exacerbated the problem to the point of him being unable to walk. According to his x-rays, he has no natural cushion in the sole of his foot so when the farrier starts digging, he's taking away whatever small amount of exterior cushion Tuffy had. I've had other horses you could dig out half their feet and it wouldn't effect them but it doesn't work for Tuffy.
There's a difference between exfoliating and trimming the sole. Exfoliating is when you stop when you hit live sole. If you scratch it and it looks like a candle wax, you don't cut it. If it looks like drywall and easily scratches with a hoof pick, then it's going to come off anyway. If you use the live sole as a guide, you'll never cut off too much.