At this point it's been going on for about four months - I hate to say it, and I know how it sounds, but you need to be a whole lot more aggressive with this kind of thing. After four months of basically no treatment, almost anything is going to be more of a problem than it initially was.
I'd suggest you prepare yourself for a long layup of this horse with some time spent every day treating, rehabbing, whatever is needed and then a gradual return to work, which from your description, that isn't what you did.
And I stand by my previous comments about longeing a lame or even a rehabilitating horse. If a vet recommended longeing for a lame horse or a horse being rehabbed from lameness, I'd be very wary. Sometimes one has to for a minute or two in order to diagnose, but beyond that, no. It's constant turning, no matter what the size of the circle. I can't think of anything that would be good for - though some minor things might heal in spite of it rather than because of it. Straight lines - either walking in hand or riding at a walk, are the usual first steps. Depending on what was wrong and how severe it was, the gradual, stepwise return to work might be two weeks, or two months, or six months....every situation is different.
As far as the lump on the jaw, that needs to be seen immediately. It could be a fracture, abcess or infection, they're not always hot or soft or firm.