I can't answer all of your questions but the third one was answered by the game. In (SPOILER) the cutscene where young Xehanort pops up and talks to Riku before the boss fight, he explains to Riku that he was the one the organization really was after, which is why he also got the change in outfit.(If i remember correctly, the sigil was the reason why they could track them through the sleeping worlds) 
  
Answer to number 1 is that The 7 keyholes are 
Destiny Islands 
Traverse town 
The Grid 
Quasimodo Land 
Prankster's Parade 
Country of Musketeers 
Symphony of Sorcery 
  
He opens the keyhole in Traverse town so he can go on to the other worlds(I think this ismthe case because at the end of the game, Sora opns a keyhole to travel back to the dream world in the mysterious tower) 
  
For #7 He could have possesed a body(Ansem did it with Riku, why not?)