"So, what do I do with his mane? How short? Do I band? Just leave it hangin? Do I want to cut it (cringe) so that there's a super straight line or just pull as normal? "
Pull it as needed, then band, be sure that you make each section even. I would practice before the show too because even though it looks simple it's not. You have to pull down the entire time you are wrapping the rubber band around the hair. Trim it completely straight after you have banded. I usually measure the length of my four fingers sideways. Err on the side of being too long, you can always go back and trim later.
I like to put a sleezy on the horse the night before the show, it makes the mane lie flat and keeps your bands from getting messed up. Take a bag of rubber bands with you for the show, and take a long a letter opener, you know those little ones like this: http://www.dollarimprint.com/my-lett...FQ5ZHgodNngcIA
They help you in the morning if you have to quickly pull out some bands, and re do. They are great for after the show, the bands come right out without damaging the mane.
"What do I do with his forelock? Braid it? Leave it hangin? Tuck it to the side under his bridle?"
Whatever looks best. I usually leave it hanging.
"What do I do with his tail? Keep it curly from just coming out of his braided tail sack? Straighten it with water?"
Curly is fine, but put on some Cowboy Magic, and brush it through before he enters the arena. The only time I take a brush to a tail is before he enters the show ring. Be careful! Don't lose a single hair. You can also put some show sheen on his tail.
"I'm also going to buy a bridle for him (borrowing a saddle). Does it matter if it has a standard browband, V-brow, or ear loop thingies?"
Personally, I like a two ear headstall. But that is really up to you.
Good luck, he'll look great!