1) Getting out to a game can be very helpful. You can meet other players, including Age Verification reps. That way you can just prove to them you're 18 without having to make a separate trip. You'll also get a good idea of how the games generally play out. Many games also have rental guns, so if you're of age but dont have a gun yet, you can probably get a loaner and play a few games!
2) Going solo in pretty much any game wont help you much as a newbie. A lot of scenarios are team based, so it would be better to stick with your teammates and move around in the same group. That way you can always ask someone if you have any doubts or are confused.
3) Some teams and certain milsim games may have ranks and designations. Other games are casual that are just about playing as a team and having fun
4) I know you enjoy sniping, but almost everyone here is going to tell you to get an AEG for your first gun. There are a number of reasons here. First is that AEGs are generally reliable and dont need much maintenance. As long as you have a charged battery and bbs, you can unpack the gun before a game and pack it back after, without running into any issues or needing any internal maintenance. A spring based sniper rifle could be used the same way, but dont forget that a) most fields have FPS limits, and its likely that your opponents AEG will be shooting at the same speed and range as your sniper rifle, and b) your opponent will be shooting 15 bbs at you in the time it takes for you to shoot 1 bb and cycle your bolt, which would put you at a distinct disadvantage.
Get an AEG first and get a feel for the game...how you shoot, how your opponents shoot, how easy/hard it is to shoot from cover or to maneuver. It'll make it easier for you. Once you get a little experience, you can try a sniper rifle. If it suits you well, you'll easily be able to sell your AEG and invest in a more hardcore sniper loadout
5) Doesnt matter really. Both games are fun in their own way.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)