It definitely works with UNC. I don't see a way to access the share or file with IP.
I also don't see why it would take very long once the NIC was enabled.
I don't know anything about your IT people; they are probably very good. But, IT people (like controlling doctors such as myself) are famous for setting something up the way they think it would work and not in a practical way.
I don't see anything you would have on those work group computers that you wouldn't want others to access. I would think everything of any note at all would be on the server. That is the beauty of a domain and a a file server, you can set up NTFS permissions anyway you want either at the user level or at a group level.
I would think putting every important file on the server, setting up one domain and network with permissions would make the most sense. Otherwise every time there is an issue, the troubleshooting will be that much more difficult.
Sorry, just my opinion, and I realize I don't really know your topography all that well.
