I understand how it all works, just one question though:

"When a hero claims a "Legendary Fortune" card from the gear deck, he returns that card to the campaign pool..."

Does this mean the card is set back aside to be put back in next quest, or is it reshuffled back into the gear deck?

I read this as it goes away until the next quest, but just want to be sure.

You read correctly. When you're returning it to the campaign pool you're basically putting it back into the box but it will be available for the next quest.