From your description the owner sounds like a small minded twit who had several repeat performances in third grade.
BUT
to him, part of his responsibilities include not selling a firearm to someone he, right or wrongly, thinks is mentally incompetent. The same as he would not (hopefully) sell to someone clearly drunk or high, or a real drooling, babbling mental defective.
Forget what you know, try to see yourself from his point of view, add the legal burdens and the personal liability issues, and just see if you can understand why he is unwilling to sell to you.
I'm not excusing him acting like a butt head, not even like, sounds like he was. I'm just saying I can understand his reluctance based solely on first impression. And I don't think this will be an easy obstacle for you to get passed.
I know the difference between mental and physical problems. I had a sister with CP, she also had minor mental problems, but we used to take her to the CP center and I am sure you know, some of the physical manifestations of CP are overwhelming to an observer. And yet some of the most physically challenged, were mentally as sharp as tacks, if not downright brilliant if you took a little time to get to know them.
I understand you know all this already. I just wanted to give you a little look into my thoughts before I say some things that I find repugnant to say, sorry that I feel they need to be said, and in a lot of way don't even agree with myself on this view.
your physical problems scare us. Twitching and jerking, funny(not funny HA-HA) walking gaits, these things scare a lot of us. And our first thoughts are these are caused by mental illness. There is no second option, and because we are scared we want nothing to do with you, we want you out of sight ASAP. We will not take the time to find out about you in any way, and we will be terribly rude about getting you away from us, because we are scared.
I, not knowing you, will assume your physical problems will not prevent you from safely using a firearm.
I fully believe you have the absolute same right to own a firearm that anyone else does, and possibly more need of one then most of us who appear normal.
and this is where I find it hard to agree with myself.
I think the burden of proving yourself qualified to purchase, is going to fall upon you. Please believe me, as much as that may bother you reading it, it really hurts me to find myself writing it. I think that rather than the government first proving you are not qualified before deigning you your gun, you will have to show you are qualified before you will be permitted to purchase.
So unless you are willing to look for a dealer that will take the time to look past your physical problems, you may need to go the court route. I think you dealt with a particularly heartless fool, who's mouth is not connected to his brain, but I can see this as a problem you will face often, even with dealers that are smart enough to not say the stupid things they may be thinking.
Good luck, I hope you catch a break. You sound like you fully deserve your firearm, I'm sorry our fear puts an unreasonable burden on you.
