1. I wasn't able to reproduce the issues mentioned in that post. If you could give topology and steps of how to reproduce the issue you're facing - I will try again. Also, keep in mind that it could be specifically ZeroTier central issues that we don't have control of.
2 and 3. It might be because you're trying to install Zerotier that is not from our package manager. Also, servers are not offline, I'm able to download anything from the Package Manager. Please check your internet connection if it didn't go down recently and maybe that was the reason for the server being unresponsive.