1) itch.io will do the taxes, they became the Merchant of record : https://squareup.com/us/en/townsquare/merchant-of-record
2) ITIN is needed for people not having any kind of record with the USA government. They created ITIN to make everyone pay taxes and yes it's mandatory.
3) Yes you still need to pay taxes, itch.io will help with that if you make enough money like $400 or more.