Using the spell Magic Jar. What would happen if a creature summoned by Summon Monster, or an Eidolon, was possessed...and then an attempt to dispel/banish/etc was made on said creature, and the step was successfull? Would it even work to begin with?
Nothing about Magic Jar says it transcends the planes. It has a Medium range. So if you are on another plane, you are definitely out of range. There is very specific wording about what happens if the host body dies or if the spell ends while you are out of range (you die).
But if those things haven't happened yet, then you have until the end of the duration to figure out how to get back to your own plane and get within range of your body or your receptacle, and if you can't, then you die when the spell ends.
So if the original host creature can be subjected to some form of dismissal or dispel magic, the possessing soul from the spell goes along for the ride rather than preventing the dismissal. Is that the correct assumption?
The rules don't really say, if the soul travels or not. What it would not do is prevent the dismissal. Either the soul goes along for the ride or it stays behind when the creature is sent away by dismissal, dispel magic, or the duration of the spell(summon monster) ending.
If I were the GM and I decided that the soul went along for the ride I would let the player know if he was a caster because I would think his character would know. That way he can decide if he wants to use magic jar.