In summary:
/overlay/upper : alias for files that have been changed
/rom : alias for files that are in the default ROM.
The overlay file system uses the file in /overlay/upper if it exists. Otherwise the regular filesystem uses files from /rom. Erasing a file places a special null mark in /overlay/upper, which causes the file to no longer appear in the regular filesystem-- though the ROM copy continues to take up space in the squashfs ROM, and it can be found under /rom and restored with a simple copy back to the regular filesystem if desired. There's little reason to poke around in /overlay.
But as @anomeome said, there is no /rom/etc/config/wireless. It is built from a script, usually only once when the router is booted the first time after installing OpenWrt. If /etc/config/wireless exists, (as would be the case after the first boot), the script does not change it. So you would move your config to a backup file then run the script to get a new default file.