Hi there, I seem to have the same problem (wrong Jaheira shows up after having left with Meronia). I searched through this topic and a few others, but couldn't really identify a solution; I did manage to get the BG1 Jaheira to show up dead on the ground, but that's not really the effect I'm going for!
I've attached the .gam file from my save (saved after she leaves but before she returns), along with my log file of installed mods in case that's relevant. Also, the version of BGT I have installed is 1.07.
I'd be very grateful if someone (Ascension?) could fix this for me please - thanks!