And this could precisely explain the discrepancy between RJ stating that the killer is kind of obvious, but the majority being wrong (and the majority actually thinks Graendal did it).

While many fans might believe that Slayer is just some random freak, who can use TAR in a strange way, RJ likely considered him as a top assassin with special gifts from the DO and highly dangerous, thus making him the obvious suspect even at the end of TFoH. IMO, anyone you thinks that Slayer is unimportant or incompetent should re-read the series. The clues to figure out what/who Slayer is, are definitely there, also in the first five books.

Besides that, I figure the DO can't (or has at least a much harder time to) transmigrate a soul in TAR, except if he is linked to this soul; Asmo probably lost this link, when Rand severed his connection to the DO in the end of TSR. TAR is basically the intermediate world between Real World and Sould Pool. Otherwise the souls of the Heroes would be in constant danger to be grabed by the DO. Doesn't seem likely to me.