If someone were coming back to the game say the first or second week of October, is there any chance they could still participate in this?
They could sign up and take their chances at first round bye.
Every tournament has some byes during the first round. Usually more if it's on the 128 man bracket than if it's on th 64 man bracket. Everything is by random draw and I won't play favorites to "let" someone have a bye. If they didn't get a first round bye and weren't able to duel by noon CDT October 3rd they'd forfeit the round and be eliminated. If they did get a bye they would have to duel during the week of October 3rd, completing a duel series by at least noon CDT October 10th or they would forfeit and be eliminated.
If I had hard dates in advance I could allow a 48 hour extension to complete the duels in either case, giving until noon Tuesday, October 5th or 12th depending on the bye situation.