If Lan dies, it would probably fit better during the LB, but not right off the bat (that doesn't sound later enough for him to have any sort of involvement with Isam, notably - which it's solidly implied will be involved with Perrin in TOM). Lan and Isam having no face off (not necessarily in battle, though possibly) of any kind before the end sounds pretty much like Morgase and Galad not meeting, Moiraine and Thom never reunited etc. to me.

It sounds possible, perhaps even likely, that it's Perrin/Faile and the Bordermen who will go to reinforce Lan, and that sounds very unlikely to happen in TOM, when Perrin rather will go to the big "reunion" at the end of the book.

My theory remains also that the most beautiful "full circle" for Moiraine would be for her to pay back Lan's loyalty, and the fact he's put his personal war on hold to be at her side and help her with her personal mission for 20 years, by telling Rand at the end of TOM that she and AS volunteers are going to join Lan. That would also repay the fact the AS failed Malkier decades ago. Moiraine's return to second Lan in his war would mean a great deal to Lan, and it might resolve the Nynaeve-Moirain thread too, if Moiraine tells Nynaeve: "I know your duty is to remain with Rand though you'd rather be with Lan. I see your sacrifice, and perhaps you understand all those I made too. I will go to help Lan for you - we have both worked to hard to keep him alive to let him die alone now. I'll bring him as many sisters as I can gather. Stay with Rand, it's you he needs now, not me. I would have liked to be at his side at the end, but it's Lan who needs me." I think Rand's refusal to send help to Lan in TGS is possibly in part because the people meant to reinforce Lan's army aren't "ready" to go, like Faile, possibly Perrin (not convinced he won't be forced to let Faile go fight elsewhere while he does something else), and Moiraine (perhaps Loial, too).

I somewhat expect Lan to die in the LB (I'm undecided about Moiraine, but somewhat I find her death at least as likely as Cadsuane's, say), but I don't really expect RJ would get rid of Lan right at the start. He would have wanted to milk Lan's war for all its worth first, and I'll be disappointed if the Borderlands rulers never get to see a bit the "Uncrowned King" in action, leading the army they should have lead. I don't think Lan will die (if he does, but there's instances of potential foreshadowing that Nynaeve might become a widow) until Perrin's ready to take over from him in the Borderlands - mid LB stuff, so first half of AMOL would be my guess.