Yeah, pre-ordering guarantees access to the rest of the betas.
Will probably need an email for each account yeah.
Fullscreen windowed mode is working fine for me.
Content is good fun so far, tried some pvp and that was fun too.
For 2500 favor (honor) you can buy a pvp soul for your class, which gives some pvp specific talents and cooldowns, which might be nice later on, at least when you get higher pvp rank, since each pvp rank unlocks another tier of the talent tree for the pvp soul.
Haven't tried dungeons yet, but i hear they are very complex and far from tank and spank already at the low level ones.