Her dying doesn't feel right, but I totally agree it feels like her next arc feels like Brandon gave her a reprieve she wasn't meant to have originally.

Her death would have undermined the whole "Rand is falling in madness" arc, her survival amplified how far gone Rand's mind was. Rand plan just wasn't rational: risking the Pattern to get rid of Graendal? Not all the Forsaken combined are worth that. Nothing should be. He was of course oblivious to his own insanity, but Graendal wasn't - he brought the bloody AK to the CK for the confrontation, it's like bringing a nuclear missile to get rid of one mouse (actually, it's even worse - its a true doomsday weapon which can destroy the world). The thing terrified LTT.

I think this was still her exit from the series, though. I'm not 100% convinced, but fairly convinced. Of course, Brandon ruined the amplification effect by moving her survival scene to TOM, but a lot of that happened as "unforeseen consequences" of the book split.

Why would he do so? To delay Graendal's original exit, which I'm sure was to end up as Shaidar Haran's pet all along. RJ's original idea, I think, was more like this: Graendal fools Rand, traps Aran'gar, barely escapes. Outside the palace (or already at her hiding place, it doesn't matter much) she is already planning ahead, weights the advantage of Rand thinking her dead etc. Shaidar Haran appears. She's failed with Rand, it's two Chosen she's killed now. She'll make a nice pet for SH, until she's broken and useless. This is either Graendal's final exit or she will have a secondary purpose, not as antagonist anymore but possibly in scenes of torture and such, if RJ felt the need to show Shai'tan's handywork more concretely toward the end. Eg: if at some point Shai'tan is close to seize Rand's soul, we could have a Graendal POV at the Pit, ripped out of her body and her soul directly in Shai'tan's hands where she'll suffer forever - to show concretely the "physical death isn't the end" horror that might await Rand.

Why delay Graendal's exit?

Because of the way the book was split by storylines. When he splitted things, Brandon began by moving the final confrontation with Mesaana from the night of Rand's epiphany to a few weeks after. He needed that to center the TOM Egwene chapters on. She needed to appear, but aside from Rand's visit, discussions, an encounter with Elayne, Egwene just didn't have much until TG began. She would have had only those, you know.. dreaded "do nothing and talk" scenes. So Brandon postponed Mesaana's demise, made Perrin come north twice instead of just once, once to witness the epiphany at DM, once to destroy the dreamspike during Mesaana's confrontation. He may well have decided to extend the Bloodknives arc too (Gawyn's arc in TOM felt stretched, it would have had more impact, in the original full book, if it simply peaked after Egwene's rescue, Siuan pushed aside, Gawyn sent to Andor. That Egwene punished Siuan but kept Gawyn around felt out of character. I can't think Jordan planned to give so much screentime to the Gawyn-Egwene romance either. With the original structure, this part of the book brought the four storylines together very dramatically - a really bad time to deal with personal subplots. The book split created this quasi-hiatus in Rand's arc and Egwene's arc between the Epiphany and Merrilor - waiting for the others to catch up, and Brandon had need for "filler". That gave him the space to stretch the romance a little. In the original structure, this part of the story probably remained fast paced: Mat left Caemlyn at last for Ghenji, Perrin dealt with Elayne, Elayne met Egwene. Rand solved the Bordermen, a few developments in Tear. Then Merrilor.).

But even keeping Mesaana for TOM I think Brandon saw another problem: I'd speculate that in Jordan's original outline, with all the storylines running in parallel, Jordan just didn't feel any need to involve directly more Forsaken in Perrin's and Mat's storylines. The last stretch of "heavy drama", the final path to the Epiphany, already started with the death of Semirhage and the use of the TP, took another big step in insanity with the death of Aran'gar and the enslaving of Graendal, then the Epiphany events already brought the demise of Mesaana by Egwene. This all makes me believe Jordan planned to still keep Moghedien and Cyndane totally in the background for Mat and Perrin, showing them in action only through their pawns: Masema for Cyndane, the BA-DF for Moghedien. For Mat, Jordan already had the gholam as concrete "epiphany" threat, and beside in the post "Epiphany" phase before Merrilor, Mat's foe was the Finns, not the Shadow.

Then for Perrin, Moridin just didn't need Graendal at all. It's long been obvious he has some "relationship" with Isam (there's a backstory to be revealed there. It sounds like Isam was raised by someone, perhaps his mother or another DF, at Moridin's fortress). After Cyndane failed with Masema and Faile killed her pawn, Moridin gave up. Cyndane had no time to plot deeply. The fact an old aborted side scheme of hers (Masema) was there let her try something, but it failed and let's move on. Like Moghedien, Cyndane has other more pressing tasks no doubt taking much time (gathering the DF for TG is one we know of - these two Forsaken have been at it since shortly before the Cleansing, as soon as they were done fetching the Forsaken for Moridin, it seems). Moridin is running out of top agents for his final preparations for TG, so he pulled out Cyndane when her only "easy" solution failed, and sent in Isam with the dreamspike (a big mistake to give the Light a taste of the device, I think - the Shadow may pay for this at the BT).

That would have worked perfectly in the original structure, but for some reasons I think Brandon felt Isam wasn't enough anymore in the different context of Perrin's story being more central, perhaps because Isam escapes in Jordan's outline to play yet another role in the last act and isn't punished or anything. That would have worked in the full book, but in the context that Perrin's storyline was "upgraded" to one of two main storylines in a novel, instead of the slightly more "supportive role" it would have had, like Mat, if the four storylines ran in parallel in one book, Brandon felt that to be anticlimactic. It may be also because the end battle in Perrin's storyline was meant to be Jordan's teaser for Demandred joining the field and Brandon preferred to keep that anticipated moment for the "start of the battle of Caemlyn" epilogue, as he had no time to introduce Demandred properly in TOM. So he wished to have another Forsaken behind that battle instead, and someone above Isam, and that left him only Graendal. He just had to delay her punishment from after her failure with Rand and give her a second failure with Perrin.

And that's what I think happened to Graendal.