Aberrants - at least the more powerful ones - do seem capable of choosing to use or not use their effects. The prophecy one comes to mind; it doesn’t give portents of Doom to everybody, only those who ask it. Newton was unable to fully discern, and yet he also spared anyone casting his calming spell.
I also speculate that the mirror is the Aberrant form of Miakoda. But this doesn’t mean that she isn’t still sentient/sapient. The entire time Lacer spends arguing with Titus and Kuchen, “The Raven Queen displays intelligent thought” is never used as evidence against her being an Aberrant. The biggest argument in favor of RQ’s potential Aberrant-hood is the seemingly effortless shadow magic. It is accepted as fact by Lacer that an Aberrant can display intelligence and act with more than mere “animal cunning.”
Also, even if the Entity in Siobhan’s memory is Miakoda, that doesn’t mean it’s her Aberrant self. She was losing control of herself after the loss of her familiar and doing her flesh casting; before her Will broke, maybe she had the thought that transplanting her mind into Siobhan would save herself from the mental decay, and was lost/selfish enough to do it (imperfectly). If it is some piece of Aberrant!Miakoda, then it does support the Red Guard’s destruction of their village as a Psychic-type Aberrant, spreading from mind to mind. But if she is instead a portal to the Plane of Darkness as you think, the effects of an always hungry, devouring plane may have simply mimicked a Psychic type (especially when the Red Guard doesnt seem to take the types too seriously).
Of course, the real question we should be asking is, what kind of spells are the Red Guard casting from Miakoda’s body? I expect that extra piece of Newton could empower a curse to propagate through a whole mess of people …