The problem with a lot of split lips is that their can be one of two things going on:
1) The boil and bite has a little "tag" left over from it's initial molding that truns into a hardened nub when you form it. That usually sits right in your gums in between your two front teeth. I had a simple boil-and-bit that did that, and without fail, one quick shot to the upper lip and I'd start bleeding from it puncturing the inside of my lip. Since it was a cheap mouth gaurd, I got rid of it.
2) With the Shock Docs they have a semi-rigid inner molding/fins that sits inside the gaurd itself. If you get them too soft before you bite and mold it, it can make the upper part of the gaurd pull back and expose a thin flat ridge that will make contact with the inside of your upper lip. It will also do the exact same thing as the little tab mentioned above.
The breathing thing is always a little more akward to get used to with a mouth piece. Part of the reason why custom mouth gaurds work so well is that even if you are breathing with your mouth open (not a good idea in a fight), the gaurd won't slip out and obstruct air flow.
BTW - There are many types of grappling gloves. Standard training/fight gloves will usually be 4 to 5 oz in size. There will be enough space to use a short handwrap under neath after you've broken them in (they'll look like this
). The "Striking" or sparring style grappling gloves will usually be 6 to 7 oz in size, and will be more heavily padded in the front. Those will look like this
. The second gives you more padding up front for when you incorporate striking during your training.