All right, looks like people are done guessing, so here goes.
The biggest issue, and the egg that came before the chicken so far as my vet, farrier, and myself are concerned, is that the angle of the coffin bones is 0* to even negative, while they should be a healthy positive 3*-5*. And that's putting everything else out of alignment, stressing the joints and soft tissues, overloading the heel region, and so on. As a respected farrier on another forum kindly drew for me:
But the issue wasn't noticed until the horse came up hobbling lame. Suspected soft tissue tear, AGAIN. This horse has a HUGE history of damaging the soft tissue in her left front leg--she's done the deep digital flexor in three places, her suspensory, collateral ligaments, and still counting, apparently.
But with these radiographs, at least the most recent lameness makes sense. Corrective shoeing for now: