I don't necessarily agree with some of the things said here. Not ALL of the items you listed need to be corrected by training, or corrected at all for that matter, in my personal opinion.
Horses come in all shapes and sizes -- and personalities. My mare is a horse who isn't too keen on grooming, snuggles, and just hanging out. Her ears are back 99.9% of the time when around people, since she is a "business" horse. Basically, "Put me to work or take me back to my pasture". She is happier being and after being ridden, and is an experienced horse with lots to offer her rider under saddle. She just isn't a big snuggle bear on the ground. I'd even go so far as to say she HATES hugs.
But that being said, you can't give your horse an inch and have them take a mile. Your mare has pushed boundaries - she has probably been with people who accepted her personality a bit TOO much, and as such she tested her way to being generally bratty.
Here is my perspective:
•she pins her ears when she is turned out and stalled - Personality
•she opens her mouth like she s going to bite you when you pet her(turned out/stalled) - Bad training
•she swishes her tail whenever I groom her - Could be personality if she doesn't act on her feelings. If she threatens in addition to the tail swishing, it may be a training issue.
•she turns her butt to me unless I have treats (turned out/stalled) - Bad training
•she has her ears constantly pinned at strangers (all the time) - Personality
•will kick out if she is tired of standing still while being ridden - Bad training
•kicks in the trailer at every stop (getting a kicking chain, can't afford for her to hurt herself) - Bad training
I'd start at the biggest issues and work down, because some of these may resolve themselves if you start to show her this type of attitude is not allowed. Kicking, biting, butt turned to you... All of those are respect issues. Treat them accordingly.
Maybe in time you will end up like me, with a horse who has pretty solid training but is always pinning at people and other horses.
Some horses are just more independent and aren't big cuddlers.
This is just my experience though and doesn't necessarily apply to all horses. My horse couldn't keep her ears upright even if she tried I think, but some horses may stop pinning one respect is established properly.