Copy the iTunes XML to where your content is now.
In your case, you might be able to add your “default” iTunes path to Firefly (it can scan multiple paths)… but I haven’t tested that scenario.
Yeah. You should be able to make your firefly path “My music/iTunes” or whatever and it should pick up the files from the nas.
You’ll have to go into control panel -> admin tools -> services and change the logon user for the firefly service to be a user with permissions to the share, but it should work okay.