I can answer a few of those questions.
Nekron sent Anti-Monitor back to the Negative Matter Universe.
The 12 individuals brought back (According to Geoff Johns the only ones that came back were shown) are back for a particular reason which will be shown in the future.
Indigo Tribe stole William Hand. The everyone else did not see them take him and go. As to why they took him, we do not know yet.
According to Johns, one White Lantern has been shown, and it is not Sinestro. If you are observant you saw who it was in Blackest Night #8. Supposedly another is coming soon.
Cnat help with Halo info though.