A good news update!
I cantered Pony both ways today, alternating over and over again. Except for mistakenly picking up the left lead the first time I asked for the right lead (bear in mind his left lead is his hard lead) he got it right every time! He still feels a little stiff to the left, but he's not counterbent any more. The bodyworker said the work she did held really well, and she thinks he will only need one or two more sessions (phew). On the less good side, the trainer who was working with him was exposed to Coronavirus, so she's self-quaranteening right now, and I'm the only one who will be riding him. Hopefully now that he seems to be on the right track, we can keep it going.
"Saddle fit -- it's a no brainer!"" - random person