Bunch of N00b questions

[Augustus13]

I played Live, but have only joined RoR in the past week.
Enjoying it, but I forgot (or didn't understand in the first place) so much.

1. Communication. There are so many channels. Which channels are best for what? If I am in a scenario (sc?) what is the best way to communicate with the people on my side. Also, maybe it's just because I am solo and in T1, but there seems to be little coordination in scenarios. It just seems there is a lot of activity and I am a little isolated. Doesn't stop me from jumping in the RvR islands and wailing away!
2. Time requirements. I maybe have 30 minutes to maybe an hour to play at a stretch. Will this lock me out from upper level content?
3. Armor sets. I have enough tokens to buy some PvP armor, but is it worth it at level 12? I have to imagine that's a waste and I should save for higher levels (at least level 20, I would think if not end game).
4. How do you mail? Particularly to your other characters?
5. Mastery levels. Is there an intrinsic benefit to the mastery levels? Like some type of stat boost? I mean beyond just unlocking other powers and traits.

I have some other questions, but those can wait. Enjoying the game a lot. Thanks!