You can use a mesh mouth mask but eye protection needs to be a full seal around your eyes. Meaning there can be no space between your skin and goggles. So if your talking about using a mesh mask that covers your full face and then pulling a visor down, no. That's not a full seal. We just don't want any magic bullets bouncing around and pieces getting in your eyes. Sorry bro beans.
As long as they form a snug seal to the skin around the eyes, yes. They have to seal like snowmobile goggles and be at least ANSI Z87 rated (rating can usually be found on the inside of the lens). If you're still not sure, post a picture of what you plan to wear.
Good call out Wraith. Also you don't have to have anything covering your mouth. Loose all the teeth you want you got tons of them. We're worried about your eyes. You've only got two of those and that is really the only protection you HAVE to wear. I would recommend something for the mouth though for close combat. I've seen a lot of teeth shot out in my day. That's just me though.
Also yes you can still join either team. Pre-pay is just closed which was $10 cheaper than walk ons and got you some in game $$$ to start out with. Show up day of for which ever team you prefer just make sure you are wearing the proper clothing. Anybody out of the two sides uniforms will be asked to find different clothes, purchase what ever shirts we may have to get you in proper uniform or just not play. Don't make us be the jack ass.