Anaander Miaanai's motivation is interesting. In order for her plan as Breq tells us she understands it to make sense, AM would have had to be entirely convinced that the implants and the download would take in a host not genetically identical, that her own personality would continue to dominate, and that AM(T) would either not be discovered, or would be allowed to continue unhindered. It's a plan with multiple points of failure, no graceful fallback, and no contingencies, which seems uncharacteristic.

More interesting, though, is that AM deliberately sent AM(T) out of communication with the rest of herself. We can probably safely assume that fragments which are physically close find their identities hybridising—AM, and for that matter Justice of Toren, can probably be usefully modelled as a scale-free network. But AM(T) has no other selves there to reinforce them and correct for personality drift, even before Breq starts in on her. So my feeling is that (some of) AM expected to end up with a third-at-least actor, partly because not all of AM('s knowledge) went into AM(T), who becomes a stripped-down, rebooted, possibly even uncontaminated, version.

It's pretty clear that Breq is not correct in her assertion that the cooption process murders the victim—the whole narrative logic demands that—so what we end up with is a Tisarwat who does not fit Breq's judginess preconception of her character, an unpredictable free agent (insofar as Radchaai are at all free agents) with three millennia of political experience and some really bad poetry. And while Breq confines herself to acting on the smallest possible scale and with the shortest possible range she can manage (and it's still not very short) Tisarwat won't.