It might be a GM call, but I'd rule the swallowed opponent would simply be popped out when you unpolymorph.
And I think you auto-pass your save against poison when you polymorph into a plant. You might have to roll your save normally and the poison would be surpressed while a plant, but I don't think so.
I'd handle both the way Jdvn1 suggested. Unless the creature you swallowed as a tendriculos is actually small enough to fit in your stomach when you are back in normal form, in which I'd let it remain in there.
My DM has decided that the round polymorph ends, I spend the entire round regurgitating my swallowed foes...
Getting back to the poison question: when I do change back, do I go back to my poison-damaged self, or does being in plant form for a few minutes effectively neutralize the poison?