So like a configuration in a git repository and the router only pulls the master branch, external person pushes changes to a feature branch and merges to master

So like a configuration in a git repository and the router only pulls the master branch, external person pushes changes to a feature branch and merges to master have to be done by you. Something like this is doable. See puppet, saltstack, ansible, etc. With openwrt you could put the /etc/config directory in a git repo and have a cron job regularly pulling it. Not sure why you need an OTP display.