Arend: you Asked, and a rather unpleasant subject it is, too…
Necromancy on Athera is the enslavement and "theft" of another being's life essence for the purpose of prolonging another being's phyisical existence…
Morriel's possession of Selidie would be a "marginal" case. She has borrowed and usurped the body, but the definitive point to be asked is: did she leave Selidie's spirit "intact" in the process? If the essence of the spirit was torn, then it's necromancy. If not, if she's held passively captive, then it's an enslaving imprisonment.
Necromancy is more than an imprisonment, it is vampirism of the life essence rightfully another's, by birth.