IMO, it also doesn't seem to make much sense that the especially careful Graendal would stroll through the basement of the mostly empty Caemlyn palace - with no servants there - without any disguise, but of course balefire prepared (though for some reason not inverted), since the possibility remained that instead of Asmo, e.g. Rand had opened the door to see her (and we know that the last Graendal wants is to confront Rand in his territory).

Besides that, Moiraine apparently used very weak balefire, but it was obviously still strong enough to prevent a transmigration of Be'lal. A Forsaken like Graendal should per se use stronger balefire, especially if she might confront her arch-enemy or a Forsaken rival, precisely to exclude that the DO might perform a transmigration. And if her balefire was nevertheless just minimal, the DO could transmigrate Asmo everywhere, according to RJ's words on balefire.

To explain the how- and where-factors with balefire would mean that RJ basically said that - by chance - balefire in such an amount was used that, if Graendal had theoretically killed Asmo at SG, the DO still could have transmigrated him. This "not at SG-location" doesn't really fit a concrete and sensible where-factor, because SG was anyway never in question to be the location of Asmo's murder.