Good evening.
1) No, it is free to create as many account and chars as you want, and it is free to log ONE account. If you wanna log more than one at the same time, you need to pay for Premium Account (either with euros or buying Coin Of Luck with your adena in game from other players)
2) Hard to say, no official info, but there is activity in every timezone. Is probably more crowded in both European and Asian timezones, 1.5k+ during those timezones i believe. It has peaked at over 2k in the past, not so sure if it still peaks above that as sieges are not great entertainment anymore (to sum it up in one word, zerg) and i belive that number came from a siege day. I could be wrong, but like i said, there is no official info. I can say that you'll be seeing people everywhere in 50+ areas and honestly still a lot of lowbies too, altho many of them are probably alts of players already here so they are not necessarily new players..
3) Very few from what i remember. Tanks have combat aura (2min fighter buff to party), Elemental summoner has the buffer pony (with clarity, empower, acumen, wild magic and the 2 party buffs), Warlock has the buffer cat, with the traditional party buffs and some extras (not really sure, i think it is bless body soul and haste? someone below will give u correct info). I could be missing something here.
4) Again, really hard to say, but i believe Bishop/EE/SE are the most wanted atm at higher levels, and to a lesser extent BD/SWS.
5) There are many greeks in this server. Probably the biggest european representation from a single country, i'd say. I believe most clans went international, but there are still some clans/CPs with many greeks.
Hope i helped, good luck.