What everyone has said so far has been really great! I like that everyone is thinking on a similar track but has different things to try in an effort to see if you can sort what happened out and set up for a better future.
A few things in your post jump out at me...
First - the weather. Horses I have been around with the horsenality you describe often have an especially hard time with the weather. Before they know how to self-soothe or ask for what they need, they often nip, paw, roll over and over, wap you with their heads, or strike. It's like the weather has used up all their tolerance and they don't know what to do. We finally started blanketing ours. It is a huge pain, but it seems to make the cold days a lot easier for training. I feel like ours take it personally if you can't help, so we feel it is important to have a routine the horse can count on for these awful weather days, or not go out. Our routine is basically feed 4 cups of alfalfa pellets or something to take the edge off, then longe for relaxation, maybe do the waltz and balancera in hand, then call it a day. Keeps their intestines moving, warms them up, and provides welcome distraction.
~he is coming 6 years old...has had lots of attention and time spent on him from us in the past year since we adopted him, not lots of training per se, but trust, relationship issues, and hoof care, as he was foundered--has thankfully grown in healthier hooves--and suffered from chronic abscesses
>>>He may blame people for his hoof problems. Neglect is very hurtful to an emotional introvert horse. They seem to ask "why, why did that happen, how could it happen, will it ever happen again, why me..." over and over. We sometimes call it an existential crisis.
~he has never been ridden, don't know if that is a route we are going to take with him. I just can't picture him being ridden. He does seem a bit wild Donald. Like a horse that just wants to be left to be a horse.
>>>I hope he can find something to give him a routine and some purpose. He seems like he has a lot of questions he wants answered.
~He is often over-aggressive with the other horses, and possessive of me in the pasture
>>>We have worked with maybe 20 horses who were about 5-7 and gelded 2-5. Horses who are gelded late or proud cut are often more possessive. Stallions have strong instincts to bond and can be weird about their people. I'm sure you've heard the stories about birds choosing human females as their mates and viscously attacking the women's husbands, while also doing things to try to impress the human females.
~I don't technically do any training in the pasture, just spend time with them out there when it's too cold to do anything else, like yesterday. Just pet/scratch each horse for about 10 min. then back into the warm car.
>>>When we operated a horse rescue with 100+ horses, usually 17 in a pasture, and 60+ volunteers, all using clicker and NH, we finally had to make a rule that there was no petting in the pasture. Humans try to give attention to the ones who need it and some animals do not understand the idea of helping out the underdog. Just like human children have to be taught to share, animals start out protective of their resources and have to learn. Sometimes horses be even harder on the underdog for his "impetuousness" in accepting the attention, and the underdog will start to cling to you. Then you wind up with a lot of displaced behaviors.
~several people around the barn have suggested he is a proud cut stallion because of his aggressive rearing, striking, posturing etc. and very active herding of the mares...but he never acts out sexually...even when pesky mares are in heat, he just turns his back and tries to stay away from them. When we first moved our 3 horses there, he kept all other horses away from his 2 fillies for quite a long time. he would charge, rear and strike, bite, kick out at anyone who tried to come near them.
>>>Seems very likely that he was gelded late or proud cut. Horses who are gelded late tend to have very mercurial dispositions, changing between extrovert and introvert and high energy and low energy very quickly. We've gotten the best results by treating them as emotional introverts who need a lot of comfort, but only giving it to them after we have gotten them calmer by using up some of their energy in a very directed way. They usually love to play, but we have to be careful to watch them closely during play as they are watching us very closely and trying to impress us. We reward them for being detail-oriented, attentive, and calm. If you've ever been around a nice stallion after he gets to breed a couple mares, he can be so gentle and cuddly, like he doesn't have a care in the world. We try to get our horses to this relaxed state through work... It is hard, but after you condition them to it, they learn to seek that "happy place" on their own and will use the work like a good Vinyasa yoga class.
~he used to be very herd bound, not wanting to leave the pasture, now he doesn't mind so much, but he will get worried if one of our girls is out of sight for too long. If they have been gone from the pasture for several hours he attacks them when they get back he sticks to himself more these days, and just looks plain miserable
>>>He seems to really need a lot of emotional support and to feel like someone is a traitor if they don't give it. Seems like he needs a special friend. One of our horses was a rescue and used to windsuck pretty badly. After he got a mare of his own he did a lot better, but worse when he had several mares. He does best with a really devoted student who fawns over him. Still, he tends to try to herd us a bit when we play with him at liberty to this day.
~he was previously neglected, very untrusting, completely dissociated and it took months for us to even be able to touch, handle him consistently
~he has never been very affectionate, doesn't like a lot of touching or brushing. will back up if he feels you're being too touchy
>>>What a tough combination of needy and desperate combined with unhelpable... Sometimes if you do something very athletic like free jumping, which gets the body and the mind tired, they will come down from that and learn to soak up touch during their rest breaks. You might see if it helps. You might also try tracing his meridians or endo-tapping. Mediterranean horsemanship would likely be good for him, too. Temple Grandin talks about not liking touch but liking being compressed. He might find light touch over-stimulating. With horses like this, we find that rocking them back and forth while they stands square, like you're doing tui-na, works really well.
~he is the horse who always will go with you...ie, they are in a pasture that is at least 100 acres, and if you go for a walk across it he stays right at your shoulder, without a halter or anything.
>>>Does he have his ears back at all, like he is herding you, while this is going on?
~when Comet struck out I had taken a step back, so I was still facing him. It happened so fast, I can't really say even what his facial expression was like when he did it. I was thinking he was in a pretty good mood, and he had just really enjoyed having his face and eyes scratched, and the icicles picked off. He usually doesn't like a lot of rubbing on the face especially, but he was really itchy, and rubbing his face on my hands too. He seemed cute and foal like, a way he doesn't act very often.
>>>It does seem like that behavior was him being mad you were leaving. Most of our younger geldings have pulled that move at one point when they wanted more attention.
~he does get possessive though, so maybe when he saw I was leaving he reacted to that with his leg. for whatever reason he did it though, it just isn't good. I am often out there alone, and have to feel safe enough to at least get him up into the arena.
>>>It seems like he might have a lot of questions about your relationship. Some more structure and predictability might be good?
~figuring out how to be with him is tough. he clearly dislikes any move you make to show him you are being dominant and pushy, and we worked so hard to earn his trust, it kind of defeats that. but at the same time, he has no trouble showing his dominance, and now acting out physically. I don't want him to become dangerous...well I suppose many people would say he already is dangerous.
>>>I think free jumping could be good if he can do it physically. I have no idea if he can based on what you said about his hoof problems, but it could be a rewarding experience for him to challenge what he thinks he can do with you there supporting him.
on a side note, I suppose I also need to let the lady where we board about this. Everyone already stays away from him, and he doesn't like people he doesn't know just walking up to him. But she should be aware. I'm scared she'll say he can't stay there if he's acting out like that.
>>>I would wait and see where it goes. Of course tell her if it gets worse but you might be able to make some changes with him now that you now what he's capable of. Since the barn manager is likely used to feeding horses in large groups and managing her space, it seems like she would not have any reason to be up close to him. Most people who work around a lot of other people's horses professionally have had stuff like this come up a fair amount and would not be rattled by it unless you got her overthinking it