It was probably an administrator testing the alert command (a command which sends a message to EVERY server hosted on Minehut) to announce that they were closing Minehut for the hour for an update, however I was not on during the time so I'm not too sure
There is no way from preventing it, the only way is using the unlimited plan. Yes, it does cost credits (which are bought with money) as it helps fund Minehut.
Unfortunately you are unable to switch your server version. However you can add support for 1.8+ users by installing the plugin "ProtocolSupport" allowing you to play with other versions.
Make sure your server is under the 12 plugin cap. If it is try repairing files in the Dashboard -> Danger Zone -> Repair Files then attempt to start up your server again
Minehut needs a way to get more money, this is because the huge increase of players the last past months and need to somehow be able to afford to host all these servers
The second plugin you mentioned, "Skellet" is already on Minehut. The first plugin requires the "Sk-Mirror" dependency, which is not on Minehut, and likely won't be due to some security bugs.
Typically the grouped up people are usually bots which try to raid and crash servers. Finding things similar or exactly like something else could've just been that the person took inspiration, but I see your point
Try using the command "//paste -a" this command skips air blocks so make sure you paste it in a huge empty area. This should reduce the amount of blocks drastically. Respond back if it worked or not.
You can achieve this by doing "/whitelist on" allowing only ops to join. To add people to the server do "/whitelist add <player>" and they should be able to join.
The thing about Minehut is that it takes 0 storage on your local computer! Minehut hosts everything on their end and you can config it all on their panel with a easy 2-click plugin installer and such
You can remove your operator status by running "deop <your name>" in chat or in console, remember this will get rid of your access of banning and kicking other players and a whole ton of other commands. If you ever want to op yourself again use the "op <your name>" command in console.