Moghedien was handled by Shaidar Haran in a very similar way and got tortured and raped for an indefinite amount of time in a vacuole (where it appeared to last an eternity to her) before she resurfaced mindtrapped.

There's a 50/50 Graendal makes a return. Either this is her end, to go on as a pet and toy, never to be seen again onscreen. It would be a fitting end for the character.

It's as fitting to make her reappear as a mindtrapped pet on screen too.

We'll see. For now, I count neither her nor Mesaana out. We know the TP can heal madness in some way, and LTT knows this all too well, alas for him. I think Mesaana's fate might be determined at the Fields of Merrilor, if she's not simply to be forgotten in a cell somewhere for the rest of the story, or already executed. I wonder what Rand would advise Egwene to do with her, if he ever learns she's holding Mesaana and in what state. Several characters have had very bad experiences holding Forsaken prisoner, and yet if she's not balefired Mesaana could still be transmigrated. I just wonders what LTT thinks of the whole issue of balefire now. This weave was forbidden when he was in charge, and it's extremely likely he was directly involved in forbidding it... Rand might well think the same way Cadsuane does about balefire now, ie: never, ever use it as it harms the Pattern.

In any case, Shaidar Haran might already have come for Mesaana already. We know he already knows of her downfall. She might have been dealt with.