I do not understand the question why you want to use OMV from terminal when it is running on Raspbian - use Raspbian from terminal to access your files.
I am no familiar with that software so I cannot help there.
Normaly you can change usernames directly with usermod - arch wiki has some command examples if you do not want to go through man usermod.
Standard is to use port 80 for http requests. Then you may want to use https (443), FTP (21) or SSH (22) - depends on what is setup on the machine. Since OMV uses web interface then port 80 is probably the only one you need. Is there some special functionaly you want to enable on your device other than web GUI?
Setting up fail2ban is probably a big task. I may not be able to help with the details when I do not know what should be the target use case.
As far as I know ufw is designed to operate with concept of IP addresses and not MAC. You can set up iptables rule or create a before rule for ufw - see link.