itch.io

Yes on discord there is a link to the old versions.

No, If you buy on Steam you will play through Steam, if you buy through Itch you will download through itch.

There is no content beyond season 5. The team started a remaster process due to the first seasons being a mess and to get the game onto steam. Steam didnt approve of season 1 redux due to rules and so it was released on itch. Season 2 of the remaster will be released on Steam, and is a continuation of season 1 redux

Yes each season is its own separate thing