Do the Primals Know Each Other?

[Allyrion]

Okay, so for the source that she retained her will, it was actually Thancred who said it.
It was still matter-of-factly said though, and doesn't seem to be something we should doubt.

It's the first cutscene from the quest, Eyes Unclouded (Princess Mononoke reference? I think so!)
I'll transcribe it -

PC explains events.
Minfilia: Iceheart used her own body as a vessel for a primal's soul?

Y'shtola: Master Louisouix's writings make no mention of such a possibility. Can we be certain this entity was a primal?

Papalymo: As certain as we can be that Good King Moogle Mog XII was a primal, I should think. Both were ostensibly summoned.

Thancred: Let's not quibble over definitions. Of more concern is the implication that Iceheart retained her will even after she was possessed. We are talking about a mortal wielding the power of a primal!

Yda: It can't possible be that easy, can it? There must be some sort of sacrifice required. Or maybe she's just special?

Urianger: What qualities this woman possesseth, I know not, but full sure am I that she was groomed for this role.

Then Urianger basically says it's probably the Ascians, since they're the ones with something to gain from summoning and dissemination of a Primal.
Moenbryda, then, says that Shiva was certainly a Primal or something very similar to what we've been fighting, due to the readings.
Y'shtola surmises we need to relook at our past encounters.

Actually quite interesting to look at them talk about Louisouix and this process without knowing that he did something similar himself.
I have little doubt, from how that dialogue goes, that Iceheart had control over Shiva.
It sounds more like a transformation (Super Saiyan-esque) than anything else.