I think RJ left not much about Demandred's role in the LB, because it was all clear in his head, and I suspect in his "narrative", he used Demandred as the main general of the Shadow facing Mat, without going much into details.

I think it was left to Sanderson to decide which army Demandred would have as the clues in the notes were few (eg: Demandred's role in sending Taim to Rand, etc. which indicated that Demandred was behind the BT), and Sanderson found the idea of having him get the BT/BA contingent "underwhelming". So he came up with a different idea and sold Harriet on it.

One of the reasons for my belief is that Brandon was about to "pull another Androl" with this and wrote a lengthy back story for it, which Harriet decided just didn't fit.

The back story itself is very "Brandon-like", which makes me think he created the whole arc himself, digging here and there in the books for elements he could use and make appear like foreshadowing... the jumara, the throw-away remark of Lanfear about another sangreal more powerful than Callandor, the fact all lands seem to have a prophecy, a lover to nod at Ilyena etc. A whole lot of recycling

I think the changes RJ made to his initial Taim-Demandred idea were not to be all that major in the end, in the sense that there were many clues that an AOL era "professional" was the mind behind the Black Tower, which was way too efficiently organized to be the sole work of Mazrim Taim.

I think the change of mind RJ had was that he decided a man like Demandred would have higher ambitions than setting up a simple power base, and he seemed to find the way Rahvin and Sammael exposed themselves foolish. He decided Demandred would plot from the shadows both to backstab Rand as late in the game as possible (by stealing his army....), and to become Nae'blis. So a rivalry with Moridin.

I think it's the main reasons why RJ determined that having Demandred be Taim wasn't such a great idea, and it was much better to have Demandred just pull the strings at the Black Tower without exposing himself. He created that way a rival for Demandred in Taim, which nicely mirrored the situation between Barid and Lews Therin, only this time Barid was the one envied. This also let Demandred manage to backstab Rand a second time.

One of the big weaknesses of Brandon's version of TG was that instead of fully using characters and groups and features already in the story, he went for Hollywoodian spectacular and ultimately utterly impersonal and boring, tedious even... all these blow by blow descriptions of the "the big picture". RJ would have gone for chaos, and staying close to the characters (who can't see the big picture...), including the villains. Like Dumai's Wells, like Malden, like Cairhien... like all his battles etc. The big OP feats were... ridiculous. Undramatic. Like movies that put all the money in the VFX and forget a good script and good acting.

The scene at the manor in KOD, with Rand finally channeling like LTT in combat situation, and the circles of Asha'man/AS rapidly copying him is ample proof that Demandred didn't really need the whole strength of Shara behind him, and would have done much better with a small army with vastly superior training than their foes, and massive, massive armies of Shadowspawn.. He would have wreck plenty of havoc on Egwene's armies and the armies on non-channelers before Mat took over with only the dreadlords from the BT in circles with the turned sisters and the survivors from the Black Ajah. The full contigent of WO, the Windfinders not required for the weather, the surviving Asha'man, the AS would have had a sizeable challenge facing Demandred behind the lines with Taim as his field commander.