afaik all OpenWrt config is stored in /etc/config, everything else is created from that.
Some specific packages might not do that but they are not common and if you use them you will know because you had to connect with ssh and open their config file in their custom location.
All packages you set from UCI or Luci web interface have their config in /etc/config.
There are also some files that exist for custom user configuration, and of course the self-signed certificate and ssh keys
By default there is nothing in /root and I'm not sure why you would want to create non-root users since doing most things requires root or a different user from the "users" on OpenWrt
I'm not sure there are drawbacks of cloning everything like that, but I would still try to do it "smarter".
There is something in /root for me because I install OpenSSH and it creates /root/.ssh for authorized_keys.
"Why install OpenSSH?" That's because I am used to it. If I have to handle some area in two different ways (e.g. OpenSSH plus Dropbear), I tend to make mistakes.
The non-root user too is the same one I use everywhere, even if I have to sudo or sudo -s to get some things done in OpenWrt.
Doing it "smarter." Yes, that will come later I hope. For the present, more afraid of leaving some config out and finding that the image does not work.