On Linux file systems you can use any character except NULL, and / is a reserved character.
E.g. on ext-4 "All characters and character sequences permitted, except for NULL ('\0'), '/', and the special file names "." and ".." which are reserved for indicating (respectively) current and parent directories."
Domain registration information is public and accessible via whois. If your domain registrar has privacy services use them. They usually mean that instead of your name etc it will display the info of the privacy proxy.
Not true. I have GrapheneOS with no Google blobs in a profile where I have Signal from play store (via Aurora) and notifications work perfectly. Signal itself will turn on the no google mode for notifications if not available.
That was my take on it at least. Manual way tends to sound to me the "if you really want to do this the hard way" option.