Immy would make more sense, truth be told. Regardless of the ending in ToB, Amellysan was completely destroyed with no hope in [pocket plane] Hell of resurrection;
So was Bhaal, and he came back. There is no such thing as an unresurectable villain, just read any comic for proof.
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Do you have anything that supports your theory it's Amelyssan?
Nothing conclusive. Gender. Connection to Bhaal. MO - an over-elaborate charade. Voice acting - BG3's the DM voice is also the voice of the Absolute, just as in BG1 the DM voice was also Saravok. And the DM voice sounds rather similar to Melissan.
So far as I can find out, Melissan was voiced by Heidi Shannon, who also voiced Jalhera. She is no longer voice acting.