Advice needed: automatically alternate between Client & Router mode

Installing and Using OpenWrt Network and Wireless Configuration
1

Hey all,
First time here, I've been exploring OpenWRT as a potential solution for a couple things.

I work in live video production, and would like a portable network to control an RPi and a couple laptops from my phone. Setting up the router is simple, but in some situations I'll want to join my network to a hardwired network so I can control devices that are already installed. From reading the docs, it sounds like I'd want the my router to alternate between Router and Client mode, depending on whether the WAN port is connected to another router or not.

To add another layer, sometimes I'll also want to connect to the venue's WiFi, so my devices can get on the Internet. I could use the RPi's wifi to connect as a client, then share the internet connection through the router, but if I'm using the RPi to share Internet, I'd still want my router to stay in Router mode.

Here's my questions:

  1. How would I setup my OpenWRT router to alternate between Client/Router settings, depending on whether the WAN port is connected in the router?
  2. How would I set this up so I can use the RPi to share Internet without messing anything else up?
  3. Any other advice? I'd like this as plug-and-play as possible, so I don't need to reconfigure it at each event.