Someone should move this to the spoilers-section.
For 1)
Ideas are many around, although six members (and the missing 13th now) are left uncertain on purpose.
One additional member more or less confirmed by Master Xehanort is Terra (or rather, Terra-Xehanort), since MX confirmed him as his vessel.
More candidates would be maybe Luxord (he doesn't fit into the traitor/too strong-willed or too weak categories Xemnas mentioned to Sora).
Another possibility would be the Riku Replica and of course Vanitas, who already has the golden Xehanort-eyes, making him very likely to be another member.
So, that would make four possibilities in one stroke:
1. Terra-Xehanort
2. Luxord (or his human form)
3. Riku Replica
4. Vanitas
and three more we don't know yet, although one has to point out that it would be very weird to have Marluxia and Larxene in there, as they were traitors or Demyx because he was too weak.
Some people also like to include Dilan and Even because they were said to be still unstable in DDD after they were revived.
For 2)
Xigbar, Xemnas and Young Xehanort goaded Sora into sleeping so that he would be exposed to the pain of the suffering ones connected to him and continued to make him insecure (like Xemnas' speech in the Grid) in order to weaken Sora's heart.
Data-Naminé in Re: Coded warned that if Sora would be exposed to the immense pain of Xion, Roxas, Ventus and the others unprepared his heart would falter and fall into the darkness.
This was exactly what happened in DDD because Xemnas and the others of the new Organization knew of this fact too and used it to their advantage.
After defeating Xemnas, Sora had used up all his energy and his weakened heart collapsed, as was anticipated by Young Xehanort.
His heart started to sink into the darkness, but Ventus' heart, despite being injured, came to Sora's aid and Ven let Sora borrow his armor in order to protect him from the darkness. It worked for Sora himself, but the armor got possessed by the darkness instead and was moved like a puppet, that's the final boss Riku then has to face when he dives into Sora's heart to save it and wake it up.
For 3) He just took the role of one instinctively because of his desire to protect Sora, this is explained by Ansem, Seeker of Darkness before Riku fights him.
The Anti Black Coat is in fact the Nightmare version of Riku's "Spirit".