Magic Jar and Shapechange

[Byakko]

Part 1:
If you use a polymorph spell, then possess another creature, does the polymorph effect stay with your body? What if you cast it after you've already possessed the target? What happens when the possession is dismissed?

Part 2:
With the above in mind: the Shapechange spell allows you to change shape each round as a free action. What happens if you change shape, possess a creature, change shape again, return to your own body, change shape again, etc? What if Shapechange is initially cast after the possession?

Addendum:
If the polymorph effect also affects your mind or mental stats, does this alter the answers given previously?

[Byakko]

No one willing to take a stab at this one, eh?

(and yes, this is relevant to an actual character of mine)

[Rakshaka]

I would say that the Shapechange does not stick, but this goes way down the line of theory-crafting. There is no RAW argument to support whether your body remains enchanted with its present spells and effects or if such things ride along with your soul. I am inclined to say that your body is what is enchanted, so Shapechange and any other spells you have up wouldn't ride along. Note that it says, "You can't choose to activate the body's extraordinary or supernatural abilities. The creature's spells and spell-like abilities do not stay with the body." I would take this to mean that even if you had Shapechange active, the new body wouldn't allow you to use the power on it. Again, more along the line of theory-crafting than any actual written rules.

[Byakko]

Yeah, this is going to cause all sorts of table variation, I know it.

That line about not being able to activate the body's extraordinary or supernatural abilities is just saying you can't use a possessed creature's special stuff, imho. So if the transmutation spell you cast grants you certain additional abilities, you should still be able to use them even in a possessed body.

The tricky part is whether the spell follows the caster or sticks on the body. (or both?) Either way causes some pretty major issues/strangeness.

[Purplefixer]

Transmutation affects a physical subject. I don't see anything here in RAW, but for my money, RAI is that your Giant Form mage crumples to the ground with a soft and useless sigh while you inhabit the body of whatever nasty you just jarred. Nor does Mage Armor go with you, nor does Bull's Strength, Analyze Dweomer, or any other effect on your body. The mind/spirit is its own doohickey.

[Ravingdork]

I have asked extremely similar questions a number of times before.
I've not had a firm answer to any of them yet.

Ongoing Spells and Magic Jar
Magic Jar Questions
A Tricky Question: Magic Jar and Other Enchantments
At What Point Are You NOT Considered "You?"

I strongly encourage people to FAQ the opening posts in the second and fourth links, as they cover a lot Magic Jar's ambiguities.