I totally understand so much of what you have posted! I have a poorly bred TWH that I ended up with (didn't choose). He has always paced and it is as rare as a blue moon to see him do anything else. He does trot, but ONLY when he is a fireball straight off a trailer and into a new pasture.
I was never big into gaited horses (actually hated them to be honest), so I was okay with the pace. It was really smooth so I didn't care. However, he absolutely won't canter. He is just challenged. I had other TWHs that one would do RW and sometimes trot but cantered normal. The other I was starting. He trotted (never got past a handful of rides before he was bought) and had the most wonderful rocking chair canter.
Recently I've read Anita Howe's book on gaited horses. It was crazy into biomechanics at first and explained why the pacey horse is the way he is. It was a huge eye opener for me. Pacey horses are dorsal dominant, so their topline muscles are the ones they primarily use, but their abdominal muscles severely lack conditioning. The use of the dorsal muscles are side to side motion (pace) and usually will go with the horse hollowing out his back and putting his nose out and head up. The abdominal muscles are more diagonal and are necessary for trotting and cantering (also needed for a good flat walk and running walk).
So, to develop a good canter you have to work on conditioning the abdominal muscles and getting the diagonal down. I've been working on this with backing up on the ground and doing lots of flat foot walking in the round pen. I even saw my horse do his running walk when I was doing groundwork after a couple sessions of conditioning!
If you want better details, I'd look into Anita Howe's book/dvd. I've been super impressed by her so far. I can't guarantee that he'll start trotting, but you can at least get a canter and then possibly (since his abdominal muscles are toned) he'll pick up the trot.
I'd avoid PURPOSELY trying to pace more. It will condition the wrong set of muscles, not to mention that the pace is one of the least balanced gaits and could be very dangerous with doing jumping (in my opinion).