

Download NatureSetup.exe on the Downloads page, run it and sign in with this account. The launcher installs Java, Fabric and the client.
Yes – download the .jar and put it into the mods folder of a Fabric 1.21.11 profile together with Fabric API. Your Minecraft account must be linked to your Nature account (link it once in the launcher).
Open Utilities → Device Slots and press "Move to a new PC". Then sign in to the launcher on the new PC. This works once a week.
Press Right Shift. You can change the key in the menu settings.
Paste the crash log into the Crash Log Analyzer under Utilities – it points out the most common causes (missing Fabric API, wrong Minecraft version, too little RAM, conflicting mods).