itch.io

Correct. If there is a new update, just go to "My Library" and click on the game and it'll take you to the most recent one.

Correct. I bought it around a year ago now before this build had been released, when it was essentially a different game, and I still have full access to download it even though it's gone through one or two 'remakes' since then. Afaik the only way you'd have to buy it again is if they decided to abandon this one to make a whole new listing on itch for whatever reason.