Growing the mane
 

After reading some other posts this may sound odd to some people, but I do not show and I prefer a full mane- the longer and thicker the better! My 2 yo Morgan, who I've had for 2 months has a lovely mane, until about 2/3 of the way up where it has been rubbed or possibly chewed off by a stablemate. It has grown about an inch and a half since I got him. Is there anything I can do to help it grow faster, so it will catch up with the rest, or should I pull/cut the other part to make it even? Will brushing it do further damage? Or should I just leave it alone?

Just leave it alone. The more you mess with it the more damage and hair you will lose. If you want to help the growth process a bit you can apply some MTG to the area. Which will help encourage hair growth. Just be sure to wear gloves when doing this cause it is rather greasy.

Mane and tail works great. You can put conditioner or detangler in it. Once the hair is long enough braiding it can help it grow faster. I did this for an Arab I was leasing and after a few months he had a gorgeous mane!
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I've had a lot of luck with MTG and lots of braiding.
I would not leave braids in any longer than a week. At that point it does more harm than good.
I make about 6 braids, give or take depending on the horse, and try to let them out as often as possible to breathe and move around; the horses appreciate that.
I've also been known to feed a handful of ground flax in the grain.

Both my previous horses came to me with mediocre quality manes and tails, constantly improved and looked awesome while I owned them, then looked crappy again within a few weeks of being sold, lol...

Yeah I have heard that MTG works the best, but I am sure other things work good too. If you braid the mane I would just do regular braids that go straight down. I would redo them kinda often so the horse does not rub too much and break the hair on the top.