Regarding the firmware update:
- If firmware update would be done "without keeping any settings", all previous information in the router would be erased.
- If firmware update would be done with "keep all settings", all information from the router would still be erased, apart from "/etc/config" directory (i.e. so that router would have new functionality, just with old configuration)
Therefore, in your case, it would be best to create your own file in "/etc/config" directory and store your desired information in this file.