Well, what caught my attention in this update was the difference between an 'embodied' soul and a disembodied one which seemed interesting but my main curiosity is what apparitions actually are, how they exist and what their relation to other aethyric denizens is. Though now that I spell it out we'd likely have to observe them at the moment of manifestation to actually get any answers, so it's a bit of a redundant question.
This would tie into the general theory of how and why the Warp gives rise to various forms of life. It's generally understood that the emotions of living beings, who have a biological and a spiritual component, gives rise to life forms in the Aethyr, and those are solely spiritual. This spirit-only nature gives them all kinds of strange and often terrifying abilities and means they lack many of the weaknesses of flesh and blood, but the cost of that is that they are foreign to reality and cannot exist in it without an expenditure of magical energy.
For Apparitions specifically, the general theory is that the fears of magic-wielders have more metaphysical oomph than of 'normal' people, and enough of them have had enough of the same fears that it created an untapped niche of spiritual energy. And either something filled that niche and became Apparitions, or something took advantage of the niche and created Apparitions, or that niche somehow spontaneously manifested Apparitions.
The exact lines between studying this and studying Daemonology are extremely hazy and possibly don't exist at all, which is part of why the Golds were so cagey about it all.
With Instinctive, does that mean that if we summoned it before a battle, and a friendly wizard casts a destructive spell, the Rider will go after the friendly wizard (unless Matilde yanked it back)?
Yes. Presumably you'd get in the habit of warning any friendly Wizards you're about to share a battlefield with to not be the first one to throw a destructive spell.
A question for @Boney : Would a Cat-form Ambush still comprise of a single stab and fade, or would it be a full-on pounce and maul before fading?
I don't know what options I would have given if a different option had won previously.