1) there is only 1 server for now. You don't need to be online to read your email as well as you don't need to comunicate with server to make your turns (issue orders to units)
2) 5) 6) No. A unit rarely lives longer than 2-3 turns while in combat to require any additional supplies. The only fog of war like feature is Secret Allies concept - you cannot be 100% sure that the enemy won't get reinforcements in a particular region unless your enemy has disclosed all his allies(either because he was confident or you made him to disclose).
3) I guess no, the only source of stratgic information is the "Strategic Screenshots" section on MA site
http://www.massiveassault.com/strategic_screenshots.php
But they are really usefull as a Tip&Tricks reading. You can also read forums on matrixgames for bits of gameplay info.
4) They ARE oversized
The largest map can be crossed by sea transport - the fastest unit in 6-7 turns