Long toes can definitely lead to coffin bone rotation. Whether it's a too high palmer angle(the angle at which the coffin bone has from the ground. Being completely parallel with the ground would give you a PA of 0. You want a slightly positive PA, but it varies horse to horse) or a negative PA, meaning the coffin bone is tipped upward. Usually when the coffin bone is tipped upward, you'll see a "bull-nosing" effect where the hoof wall looks like it's bulging out. In reality, it's the laminae detaching from the hoof capsule.
The hoof capsule is like the outer shell of the hoof. The coffin bone is not the hoof capsule, it's right smack dab in the center of the hoof capsule protected by laminae and the sole. So when the toe stretches out, the laminae(think like really strong Velcro holding everything together) detaches from the hoof wall and allows the coffin bone to rotate. The sole is the only thing protecting the coffin bone from the ground. That is why it is so so so important not to touch it besides exfoliating sole. That sole is there for a reason. The hoof knows what it's doing. When the coffin bone sinks, so does the sole. That's where you get the "flat-footed" look from. The hoof will gain natural concavity when the coffin bone is in its proper place. It's like a cradle, hugging the coffin bone from underneath. Paring away sole to make it concave does nothing to aid the internal structures of the hoof. It just brings them ever so dangerously closer to the ground and makes it so much easier for the coffin bone to penetrate the sole. That's why your horse is lame after a trim. I imagine it's not much fun to bear most of your half ton weight on a bone only protected by a couple pieces of cardstock paper.
This is all just how I understand it, so some of it may be incorrect(probably a lot of it LOL) but I've personally had the experience to go through this, and I still am. It's a lot of stress knowing that your horse isn't comfortable, and having to wait. Because like anything, fixing the hoof takes time. Not externally, of course, but internally. While everything looks nice on the outside within just a couple trims, it takes a bit longer for the internal structures to mend and get better. That being said, your boy's not too bad off. I would find a different farrier, forbid him from touching the sole, and have him back that toe up and bring the heels back just the tiniest bit. I would also be religious about keeping your boy's feet clean and treating them to help with the thrush. I personally use apple cider vinegar, but Pete Ramey has found great success with "50/50 mix of Triple Antibiotic Ointment and Athletes Foot Cream (1% Clotrimazole) (for humans; over the counter at any pharmacy." I myself will be giving that a try as soon as I find thrush bad enough to use it. My boys just get a little icky when it gets wet so I use the ACV as more of a prevention than anything.
I know this is A LOT to read. I didn't mean to write so much.
And like I said, take everything I say with a grain of salt.
I'm still learning myself but it's a great thing to learn and is just amazing to see how much more comfortable your horse could be with just a little rasping.