Well, I have yet to see that horse the "rears very nicely"!!! And "No!". This horse is not confused. He knows exactly what you want -- I'd bet the farm on it. The only thing you are right on is that this horse is not trying to hurt you. He is simply saying that he does not plan to back up -- no matter what. And if you get in the way, well, too bad.
ALL of the methods presented will work on a green or confused horse. I do not think this horse is either of these. I think he has been asked enough times that he know what is wanted and has made up his mind NOT to back up -- no matter what. It has become a 'mental block' that he has decided to make a stand over. How it originally started or what 'cues' a person gives now are immaterial at this point. The battle lines have been clearly drawn by this horse.
I have seen many just like him, including one that had been sent to 3 different professional trainers and still would not back a step without a person in front of her and another that turned into a full fledged flipping horse that would throw himself over backward the instant a rider asked him to back.
I have refused, for many years, to get on a horse that refuses to back up. I knew a man some 45 years ago that was killed by horse falling over on him and the saddle-horn broke a rib and it punctured his heart. I have known of others that were badly injured, so I just do not get on them until I know they are going to back up instead of locking up.
I have a list of procedures that I go through. Here they are.
First -- I back the horse from the ground. They usually do that just fine because someone has already done that; but, I make sure they will, anyway.
Second -- I put the horse in driving lines and spend a short time driving it. I NEVER run my lines through the stirrups; I always run them through 2 big rings I tie to the back rigging dees of a western saddle. After getting the horse 'bridle wise' and driving well going forward, I drive it into a corner, stop it and keep pressure on the reins, hoping for 1 step back to relieve that pressure. Should the horse back up a single step, I take all pressure off. Usually more driving will result in the horse reluctantly backing -- but better than not backing at all. It is a good starting place. If the horse just bows his neck and refuses to back, I go to plan 'C'.
Third -- I put the horse in driving lines PLUS have a second person back him from the ground at the same time. Sometimes it works, but sometimes it does not work because the instant the ground person steps away, the horse 'dummies up' and refuses to take another step back.
If this is the case, putting further pressure without additional training will only cause rearing and/or flipping.
Forth -- I will teach the horse to accept being 'bitted up' in a snaffle with a snug noseband. I always use side reins with an elastic link in them (not just the little doughnut). Even then, horses that are this resistant can still fall over backwards or 'sit down' and roll over backward.
I get them to walk, trot and lope forward while bitted up. I ask for back-ups while bitted up. If the horse backs up from pressure on the ground, I try again to get the horse to back-up in driving lines. Generally speaking, once a horse backs up in driving lines, it will back with a rider.
Finally, If a horse still refuses to back up, I check its head around to one side with one of the side reins. I do not check it up tightly, but just slightly more than it would be if a rider could barely see the corner of its inside eye. I tie this single check rein solidly. Then, I 'drive' this horse forward in a round pen or other enclosed area. After the horse is thoroughly loosened up going in small circle, I snap a lead on the bit where the outside rein attaches. Then I pull hard and steady on it while the horse's head is around partially to the one side. The horse has few options. He can fall over or he can fold up his head and neck and finally back up a step or two. It is much easier to get a horse to step back when they are bent in one direction or the other.
I have had one or a combination of these routines work on all of the horses I have re-schooled.
If a horse just backs very slowly, dragging his feet instead stepping back easily with diagonal pairs, I simply 'close the door' in front of them and close my legs on them (closing the doors on each side) and ONLY leave the back door open. I boot them in the ribs and smooch as I back them up, and THIS it what gets them to pick up their feet and step back instead of dragging themselves back. In other words, you ride them backwards exactly the same way you ride them forward. You just leave a different door open for them to go through.
The most important thing is to offer relief and release the pressure every time a horse takes a step back. You can gradually change your aid from rein pressure and leg pressure to a que word like "back" or to shifting your weight and moving your feet forward (as most reining horses are trained), but every horse should yield to pressure on the bit to start with. It is just a part of being a trained horse.