Some horses, you have to really work for their confidence and respect but ONCE YOU DO you will have a horse that gives its all for you. Not all horses are alike.
I have some of the challenges you do, in that my stallion is stalled and is only "out" when I am there to work him. Unfortunately, there just isn't any option for pasture or large turnout here, and I am sure my guy would benefit from something like that because he is high-energy and loves to play. So therefore, I do my best to indulge him.
Clipping is a fear that many horses have. And fears can get better or worse depending on how they are handled. Obviously your guy has had a bad experience, and that will take a long time to overcome. It will take an extreme amount of patience and dedication for you to work with him, if you want a horse that is a pleasure to clip. I am sure you can be successful but it will take a long time. Obviously slow desensitization and reward is the route to go. I would recommend that if you have to clip for a show in the meantime, use tranquilizers so you don't undo any other work you do. My "bad" horses are clipped at least once a week and I do bridle paths, muzzles, and fetlocks which are generally the "uncomfortable" areas for them. The ears... well I have never been a stickler for ears and I just clip what the horse will let me. I've never tranquilized or twitched a horse to clip... I just have patient marathon sessions! But that is my own personal way and it IS strenuous, but it pays off in the end. My stallion started out as a three-month-old weanling that I couldn't get anywhere near, and now he lets me shove clippers down his ears three years later like it's no big deal. But it took a very long time. Now I only clip him up once a month. "Bad" horses also get what the lady at my barn calls the "moth eaten" treatment.... I clip at them all over and they sort of look moth eaten with patches of hair missing everywhere, but all year long I clip at them with clippers so they don't forget their lessons during the long non-show season. Since you basically have to clip the minis before every show, it's an important life skill for them.
As to the attitude, I bet part of that is from boredom. Every day I go into my stallion's stall before his grooming-workout-playtime-feeding-session, he gives me what I call "cranky ears" where he faces me and pins his ears back. It's not that he's aggressive or being mean or anything, it's just his way of expressing "Geez mom, where have you been, I REALLY want out of here!" and I just wait patiently with the halter to catch him until he gives me "happy ears". I wait for "happy ears" before letting him eat his food or going out to play, and then reward him accordingly. I believe some horses develop this way of expression, even though you might read it as a "bad attitude." I would be willing to bet anyone who doesn't know my colt would guess him to have a "bad attitude" by watching his ears and the faces he makes, but he's actually sweet and playful and very bonded to me. I believe it's just because of his situation, and I don't think he's ACTUALLY MISERABLE or upset or peeved off.
To combat this "crankiness" that my stallion has, I give him a lot of grooming, as much physical work daily as I can, and I try to challenge his brain while I'm working with him by varying his routine and doing a lot of random stuff each day like learning tricks, setting up for halter randomly (on the way to the arena, suddenly ask him to set up, or whatever) and taking him for walks out of the area.
I also make sure daily that I "touch everywhere" and usually I do that while he is eating his grain. I touch his sheath (udder if it's a mare), under his tail, his belly, his ears, grab his face and pet his eyes and muzzle, and grab his tongue. I have found he actually likes me to hold his tongue while he is getting his shoes nailed on! It's his favorite! I think that it helps horses to trust you if you are constantly running your hands over them. Find their favorite itchy spots, and use itches as a reward when they do something you like (if you have trouble picking up feet, pick up the foot, give it back, and then reward on that itchy spot).
Find a routine that you can get into, that will involve incorporating as much as possible into the time you have. I only have an hour or two per day that I can dedicate to both my horses, so I have to cram grooming, exercise, bonding, feeding, cleaning, and clipper time in as efficiently as possible to maximize efficiency.
Some horses require a lot more than others, some horses you can clip once a year and they don't care. Some horses can stand in a stall for a week and don't get pent up. My horse, well he's not like that. And maybe yours isn't either. They are all individuals and have different environmental circumstances.
Get to know him, give him a chance, and work as slowly and carefully as you can. I know you will turn him back around, it just takes time and patience.
Good luck,
Andrea