I can't officially answer you, but from reading the forums I can tell you:
- If the game doesn't work, the person will be directed to you first, to try and help them.
- The person who bought your game can request a refund from support.
- I think itch.io support issues refunds on a case-by-case basis - they don't have publicly announced rules. For example, where someone typed in the wrong amount (added a zero), or where the person claimed the item was misrepresented (they didn't get what the description said they would get).
- Itch.io would refund it, and deduct it from either your earnings.
- Itch.io generally has a 7-day window before they release the earnings, so I assume the buyer has that many days to request a refund, but again, I think it would be at the discretion of itch.io support.
For the last part, I'm not sure if you can receive a payment if you haven't setup any payment methods.
Itch.io has said they're not a bank and would like developers to claim their payments within a reasonable time frame. But there is a minimum amount to qualify for a pay out. Only if you're going to receive at least $5 after deductions will Itch.io pay out the money to you. Deductions include a one-off fee for a tax interview, pay out costs, itch.io's share, payment processor fees, and taxes (if applicable). More information here