Correct. This is via Virtualmin >> Email Settings >> DomainKeys Identified Mail
Correct. When virtual servers are created, Virtualmin takes care of its configuration if DKIM has already been enabled on the system. You do not need to manually add domains to the text area captioned ‘Additional domains to sign for’.
Year is meaningless. I use the hostname, e.g. vps01, so the DNS record looks like vps01._domainkey.domain.com and in future if I migrate domain.com to vps02, this naming system offers me the advantage of two simultaneous DKIM records in the DNS for that domain. If you use 2020, you might have a collision with another server to which you are migrating the domain.
Correct. Some DNS management interfaces (AWS Route 53) want long DNS records broken up like this:
"v=DKIM1; k=rsa; t=s; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA5iUXsdYFAKne/"
"qncNIGPOWJmApXZr+tmf4sEIudFl4hpY0KWLUQLZ7IqyB1dH6Mb60we3y1TkoOksXbOtBLIrfjp5DFI2"
"KzvaQOGkTxMOSPF4J7gq98BmgdeActNli64WMZ0aOxXdePsslo6lmkenj+6Lz70QuUk0J/O7qZp4fWVp"
"u560NkJ2AYvAGvRAVkdknm4ZdE8OukLH3K3lM+EnVv/o7Y5YgU1+40KfV2Z8rauVHpONJcNciw9YwLZh"
"KLTefGUVj1F7IN5LvZNbZKz7zZitDGesVYDIbr4D20j6MGj+sGXBVOZQ8YBOOZSZnGKL5oFOKCAmbu9x"
"ln3jpj9+QIDAQAB"
Others are happy with this:
v=DKIM1; k=rsa; t=s; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA5iUXsdYFAKne/qncNIGPOWJmApXZr+tmf4sEIudFl4hpY0KWLUQLZ7IqyB1dH6Mb60we3y1TkoOksXbOtBLIrfjp5DFI2KzvaQOGkTxMOSPF4J7gq98BmgdeActNli64WMZ0aOxXdePsslo6lmkenj+6Lz70QuUk0J/O7qZp4fWVpu560NkJ2AYvAGvRAVkdknm4ZdE8OukLH3K3lM+EnVv/o7Y5YgU1+40KfV2Z8rauVHpONJcNciw9YwLZhKLTefGUVj1F7IN5LvZNbZKz7zZitDGesVYDIbr4D20j6MGj+sGXBVOZQ8YBOOZSZnGKL5oFOKCAmbu9xln3jpj9+QIDAQAB
Correct.