1) yeah the premium version updates to the latest version at the same time as the public release of the game. (Supporters still pay for and get early access.)
2) Yes there is a guide included with the downloads for the 3 different versions, or you can access guides and community helpers on the discord for free. (Guides pinned in one of the channels.)
3) Not if you do it right, but it's easy to mess up. Recommended way to install the game is to install the new version overtop of the old versions. But because the name of the app and everything has changed you may need to rely on the upload sync/download sync method.
[In the old game with saves]
You can use upload sync on you save game page (write down/record the code it displays)
[In the newer version]
Then use download sync on the load game page and the code to download your save files.
Don't get this backward, you could overwrite your saves by asking it to download 'nothing' overtop of your saves.
Yah, thats the recommended way to update. New version overtop of the old so that the saves don't need to be moved anywhere and stay safe. on PC/Mac they're stored in like appdata/ren'py tree and perfectly safe from deleting the installed game files, so you can just install a new version. Mobile is trickier, because the saves are within the apps structure, remove the app and you remove the saves.