A205: Yes, I believe they would be able to benefit from their magical items, though this wont really effect anything unless the fiend of possession is currently controlling the creature it is possessing (Possession =/= Controlling, Controlling requires you to Possess and then requires you to roll for control).
A206: Yes, a Fiend of Possession who can use soulmelds should be able to use them while possessing somebody, but see A204, however if they Control the creature they are possessing that creature should be able to benefit from the soul melds as well.
A214: No, I dont think there's a way to get that to work, you can have the Fiend of Possession control the creature and then cast the spell, but said spell would still be targeting the Fiend of Possession.
As far as Fiend of Possession cohorts go, I think it would be better for you if the main character was the Fiend of Possession, and the Cohort was the Controlee.