I still think
the Emperor coming close to SH, breaking free from the brain's influence thanks to the artifact and then tadpoling SH is the best explanation. The Intro showed people being teleported directly into the pods by the tentacles so maybe no other mindflayer than the Emperor came close enough to SH to be affected by it. (I won't die on this hill though)
But then that begs the question of if
SH was personally tadpoled by the Emp,
why is she ignored almost immediately in favor of the PC who can be a random Balduran? Larian also has the artifact magically teleport into the PCs possession if you never ask for it, too.
Well, in the end, this is a gameplay convinience since you always need to have the artifact with you, no matter who your pc is. We can certainly come up with some explanation, like that it took the Emperor a moment to decide that the pc seems to be the most important/capable person of the group (remember that he visits all companions' dreams in the beginning but then settles for the pc, who also might be SH).
Another interesting question is why the people left at camp don't fall back under the Absolute's control as soon as you stray too far from them for a longer period of time.