1) I assume she's as "whole" as Arthas felt after we shattered Frostmourne. Still technically undead.
2) Probably yes on the chains, no on the banshee form. I doubt the Jailer let her keep the powers he gave her, but if she's still a banshee-inhabiting-her-own-body then she'd keep anything that's a banshee power.
3) Join...to the alliance? You think we want her more than the Horde does? Remain imprisoned, we need to interrogate her!
4) I imagine she'll keep the Maw transmog for now for simplicity's sake.
My main concern remains that Thrall promised Sylvanas's head as proof "the horde has changed" in Shadows Rising, so either her redemption or the Horde's are off the table, and we get to find out which.
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I'd say she failed then. The "softies" (aka the Horde leaders who cared about their people and were not supervillains) are currently in a council in control of the faction, abolishing the Blood Oath and the Warchief position entirely.