IMO, a few of those things were overdone by fan expectations, and there was no real way to live up to the hype. Asmodean's killer doesn't seem to have been intended to be a big mystery, mostly because from what I have seen of RJ's perspective suggests that he didn't think the specific identity would be all that important. His recognition of his killer points to it being one of the few people in his narrow social circle, most of whom were dead (Lanfear, Kadere, Isendre, Moiraine, Rahvin, Aginor, Baalthamel, Ishamael, Bel'al) or already had an alibi for the time of his murder (Rand, Mat, Aviendha, Egwene, Lan. Moghedian). Of the Forsaken known to visit Caemlyn, there were only two suspects, and the next book made it very clear that Sammael didn't do it. "You?" almost certainly makes the killer someone from the Shadow, which gives you the motive for his death, and makes the specific identity fairly irrelevant. The important thing was that the Shadow could still strike at Rand's inner circle, even at what seemed like the apex of his rise to power (as Min would suggest in the prologue to the very next book), and could remove assets and allies. Once it blew up into the massive mystery it did, there was no real reason for RJ to make the revelation that apparently to him seemed obvious, as well as fairly unimportant to the rest of the story. In the first place, the impish delight he appears to have taken in saying "read and find out" for so many mysteries and questions would explain why he didn't come out and say who did it, and in the second place, it is just common sense when you are supposed to be promoting or selling books, not to deflate a point of interest that could make people want to buy the next one.

Moiraine was also not supposed to be such a big build up. Thom's letter was revealed in book 11, and RJ planned to have her rescued in book 12. No two successive books in the series had a longer gap between release dates, as it turned out, than books 11 & 12, and then Sanderson's decision to split the work added another year to the wait. Waiting five years for something RJ would typicalled have produced in less than three, with another book in between the two events, drew it out and built it up.

Demandred, I also don't know about, since Jordan seems to have changed his mind along the way. Giving him the benefit of the doubt and assume that Taimandred was ditched prior to the release of LoC, and that he was not deliberately jerking the readers around and changing his mind after people started guessing correctly, there are still too many bits left in LoC that can't be explained by anything other than Taimandred. The obvious explanation there is that LoC was the second-longest book in the series, and released only a year after its predecessor. The next three books were about 100,000 words shorter, and took closer to two years to come out. The next one to break 300,000 was KoD, for which RJ insisted on a full editorial review. I didn't (and still don't) know what difference that would make, but he seemed to think it was important. Maybe he was conscious of all the stuff left in LoC from earlier drafts that he did not have time to excise in the process of getting it out exactly one year after tFoH?