The "discrepancy" is internal to TGS. Brandon goes and tells us that Natrin's Barrow is famous and a remnant from an earlier era, and tin the days of Arad Doman the fortress was used as a royal country residence until 50 years ago, and then he goes and would have us believe "it's now largely forgotten" by the populace. It's not terribly realistic. Many probably forgot the place was once a royal secondary residence, but not its name or location. It would still be a famous very old fortress deep in the south... but not probably who lives there now, unless that cousin's descendants are well-known people, It would also be on every map of Arad Doman. No cartographer would omit the location of a place like that.

And no way a man like Bashere or his noble officers wouldn't know about a fortress like this deep in Arad Doman.

It's Rand saying "it's been largely forgotten" and going to librarians that overdid it and doesn't fit. Superficially it's very WOT-like, but Jordan himself would not have made a place still used by the Kings of Arad Doman "largely forgotten" merely in 50 years. That's no Manetheren left alone in an isolated area for 2000 years (mind you, everyone but TR folk seemed to know about Manetheren).

OTOH, the back story of Natrin's Barrow fits Sammael's comments very well. It also fits the previous clues about its location and the description of it we had, including in the TGS prologue scene (that features another discrepancy likely introduced because Brandon wanted to give us a taste of the location before chapter 1 when Rand wonders where Graendal is: Graendal is enjoying the sights of the mountains near a window. Earlier on we learned she doesn't like to look at nature outside - nature irritates her - and she doesn't use the rooms of the palace with windows. Another point: she compares Moridin to drawings of Rand. She didn't need drawings of Rand or at least it's odd she mentions them over the fact she saw him with her own eyes when she spied at Sammuel's lair after his downfall. Brandon's research for the Graendal details wasn't very thorough, it seems.).

I don't think it's even a remote possibility that Natrin's Barrow isn't Graendal's palace for real. It's a fairly weird notion. The only way Rand learned of it is from a near miracle. Kerb hated Graendal so much the depths of his destroyd mind still held the name and he was able to speak it before he died. It's no bread crumb Graendal dropped on purpose to fool Rand. And since it isn't, then it's very obviously her real lair... where she very obviously was since she compelled Ramshalanm when he showed up....