The config files are generated in a distributed way, and the exact methods depend on the platform and router. So, there is no single place with all default files.
For most routers the following things are valid:
(assuming normal squashfs + jffs2 overlay filesystem, mac80211 based wifi etc.)
* There is no backup copy of settings for packages installed by the user via opkg. Those settings are just in /etc/config
* For most of the packages included in the firmware image, the original settings can be found in /rom/etc/config directory
* /etc/config/network is generated by a script at the first boot after flash. No config file copy anywhere.
* /etc/config/wireless is similarly generated at the first boot. But you can re-generate the defaults with "wifi detect" command if you first delete or rename /etc/config/wireless . (The script first checks that there are no settings, so you really need to remove the existing settings file to see the output of the script. )