is that it?
I think if you remove the usb installer you get the error you have to boot with the usb installer connected and you will get OMV's WebUI, but don't login instead ssh into OMV or if you have a keyboard and monitor connected login as root run update-grub remove the install usb then type reboot then hit enter. It should now boot from the drive you installed OMV on.
Then do your option 3 before shutting down and connecting your drives, if that works you'll probably have to come back if the raid doesn't show.