Didn't read all of the others, but this is not a real unusual behavior.
Ruling out saddle fit and back pain (which I doubt), it is a 'learned behavior' with a likely 'pay-off' for the horse. I have seen horses learn this in one ride early in their training. The rider asked for a lope, the horse had a hump in its back and the rider pulled the horse up thinking they would ask later when the horse was more 'ready'. A year later, the horse still threatened to buck or humped up to crow-hop and the rider was still pulling the horse up -- the 'pay-off' for the horse is obvious by this time, but still happening.
The same action (pulling the horse up) can result in a horse with an aversion to loping. Those horses trot and trot and trot, faster and faster and refuse to break into a lope no matter what. They need the same 'fix'.
There are a couple of ways to address this.
First -- would be to just force the horse to lope on. Of course, the rider has to be confident enough and able to stay on if the horse actually breaks in two. This is not likely with this rider or the rider would not have been pulling the horse up in the first place.
The next way is to get a very experienced and confident rider to MAKE the horse go on. Like rearing, it is pretty hard for a horse to buck hard when moving forward at a good speed. So, if you can find such a rider and they work the horse hard at a lope for a while, you should be able to get on after them and the horse should lope for you, too, without threatening to buck. Of course, this rider, too, has to be able to ride the horse out if the horse decides to really buck hard.
The way I would really suggest would be to put an 'over-check' on this horse as demonstrated in this previous thread and, with it pretty snug and MAKE the horse lope, After a few strides at a lope, the over-check can be taken off and the horse will have learned that it cannot hump up or drop its head and it will lope on just fine.
Here is a copy of part of an old post where I showed how a little over-check can be simply made. It is very effective and can be taken off as soon as the horse figures out the behavior no longer works. It keeps his head up without pulling him up and stopping forward movement.
If a horse really wants to buck, I run the little nylon cord between the two ropes on a rope halter. That sets it much higher and it becomes an 'over-check'. It is very effective in stopping a horse from dropping its head.
Let me explain this better since you cannot see how the little cord runs. I take a 9 or 10 foot piece of 1/4 inch nylon cord (purchased at Walmart). I tie one end solidly with a bowline knot to the bit on the right side. Then I run it through the top ring of a halter as shown or between the two ropes of a rope halter crown-piece. Then, I run it loosely behind the saddle-horn, run it back through the halter just below the horse's ear and tie it to the left bit ring with another bowline knot.
Then, I take the loose cord behind the saddle-horn, fold it over itself (making sure it is centered and the horse's head is straight in front of him) and drop the double half hitch it made over the horn.
I cannot count how many times this little gizmo has saved my neck. I use it on every green horse when I first ride them off. I usually warm them up a little and then loosen it by taking the half hitch off the horn and just lay the cord loosely behind the horn.
I use one of these adjusted loosely when I ground drive a horse so the it cannot drop its head to avoid the bit. It also keeps a horse from bending its neck vertically in the middle (instead of breaking at the poll) which puts them on their front ends.