I've seen long banded manes, but I think for it to look nice, the mane can't be thick.
As for braiding the mane and not the tail, that (in general) seems to be acceptable.. if not preferred to a messy tail braid! If you have some time to practice, I'd suggest learning to do a running braid. I can do a nice, tight, show ready running braid in 10 minutes, if my horse stands still.
Here is an example - yes her head is turned so it bubbled a bit. Please note that I didn't use gel or hairspray, so there are wispy flyaways.
Straight side shot.
And the length of her mane, right after pulling the braid out..
And another example, on a horse that isn't black, lol.
You want it to be TIGHT to the neck for showing. Tight tight tight. And you'll probably want to use hair gel, hair spray, or a combination of the two. It's easiest to get tight if you leave the mane dirty. When it is clean, the hairs tend to slip. Obviously brush it etc, but don't wash it for a few days before the show. It can get wet etc, but no shampoo or conditioner.