Why does it say "Player does not have a Skills profile!" when trying to open the skills menu? / Why are the items in the skills menu missing?
Restart the server. You likely reloaded the server, which is not supported and will break things.
Will you ever support 1.8?
No. Too much would have to be re-coded and features would be cut.
Is there a developer API?
Yes, find the dependency info at the bottom of the README. The main class is AureliumAPI, which contains static methods you can use.
How do I uninstall the plugin and remove extra hearts
To uninstall, put the plugin back and run skills resethealth in the console when no players are online. Then immediately remove the plugin and restart.
Why are AureliumSkills commands not working on my Minehut server?
This is an issue with Minehut's server software, there is nothing I can do on my end to fix it. This issue is not present on normal Spigot/Paper server jars. You can either send them this (https://pastebin.com/1ZSeFy7K) error that is causing it and hope they fix it or preferably get a proper server host that allows you full file and console access