FIXED!
If anyone is to stumble across this in the future: to access the mobile interface of Calibre web, it's not the same port as the desktop/JavaScript web interface for the server. You need to access the content server port.
For me, going to [Server IP]:[Calibre CONTENT SERVER IP]/mobile ... worked. (For me, 192.168.0.2:8081/mobile)
Hopefully this helps someone in the future, but alas perhaps nobody will be as daft as I have been for the past multiple hours.
@saarg, thanks for your input regardless. So happy to solve this!