I agree with several of the other posts...keep it long unless the shows that you attend are of the caliber that would necessitate pulling it. He has a beautiful mane and it looks very nice long.
As for braiding it along the crest, those braids shouldn't be left in place for too long. I take them out after each class and rebraid to keep it crisp looking. It can also pull hairs out if you allow the horse to eat off the ground with the braid in.
Oooooooh his mane is sooooo pretty natural, I guarantee if you chop it off again it will poof up. Just do a french braid in his hair those don't come loose, my mare has her mane 3 inches past her neck and all I do is
1.start off by doing a normal braid but only do a few of the way down then put two of the end strands together and get one new strand of hair and braid into it then continue doing this all the way down the neck but braid close to the neck and when you get to the end continue braiding down then double the braid and you can use electrical tape or two rubber bands to secure it