The Imoen's Bhaal-essence was filter out as a result of Irenicus experimenting on, and removing her soul, right? I forget how they wrote around that in Throne of Bhaal.
Yes but the MC as well as Imoen got their souls restored which were stolen by Irenicus and Bodhi and with that also Bhaal's essence because Sarevok could restore his body in ToB when Imoen instead of the MC helped Sarevok with some of Bhaal's essence. Also, why would only the MC get back the essence of Bhaal during soul restoration?
Imoen should be dead: "Imoen was believed to have died some time before the Year of the Narthex Murders, 1482 DR, as the only two remaining Bhaalspawn at that time were Abdel Adrian and Viekang." - Source: https://forgottenrealms.fandom.com/wiki/Imoen
