An outsider is a creature usually with a body innately tied with it's soul, resulting in it's inability to be resurrected. Does this non-duality of body and soul also cause outsiders to be immune to spells that displace the soul such as Magic Jar or a Ghost Possession ability?
No, if it did, they would have explicit immunity to such effects.
Technically the lack of body-soul duality should throw a kink into certain things like magic jar, but for simplicity I generally have the spell work as intended, but twist the way or reason that it works. For instance if you magic jar into an outsider I typically have the spell work as per normal, but the outsider's soul isn't moved to the magic jar's focus, because it's inseparable from / it -is- its body. It's more that a second soul is juxtaposed and in the driver's seat temporarily.
I usually go rules as intended, even if a strict reading of outsider nature should render certain effects impossible.
I appreciate the Feedback. One of my characters was trying to get out of his Eladiron being possessed and this started an interesting, long rules discussion. I was just curious what others would think.