well, you're physically disrupting your network connection by unplugging the cable.
You probably need to restart your filemaker server services AFTER the wifi connection is established. Essentially you're unplugging the "live" connection, and it is disconnecting because there is no ability to automatically roll over.
I would seriously consider investing in a wireless router. Even if the router itself isn't connected to the internet, you can plug the mac mini into it directly, and then have your devices connect to the routers network. In essence, creating your own portable LAN.
Ad-hoc networking ( I don't believe ) has even been a really recommendable setup for FMS.