Okay so the person who plays the card "Open the Limbo Gate" gets to choose a character out of each of the discard piles (his own and those of his oponents).

Acording to the FAQ there has to be at least one character in each discard pile to even play the card.

Cool with that.

Then the person who played the Limbo gate puts those character cards in play....

Where?

All on his side? Or each on the side of the their owner?

Very cool if the first. Still fairly cool if the second.

the control of the character doesnt change. so you put your best character back into play and their worst. unless you wanna be nice and give them back the best character in their descard pile.

as the rule says p.14

"When a card comes into play, it does so under the control of its
owner , unless otherwise specified by the effect bringing the card
into play"

When a card comes into play (you could add from a hand, from a Deck or discard pile )