Yes once you buy it you'll get access to all the future versions, even if I increase the price of the game. You buy it only one time if you prefer UwU
Also the weird download library is probably because of the bundle system. I have to do the same with Bundles I bought. It's just how itch.io works I guess but once you're used to it it's pretty simple :3
About the updates, you have to download the new versions of the game yourself. You keep your saves anyway and can delete the previous version.