your write-up is highly appreciated 
for me it makes it somewhat hard to understand why you use collabora-built-in if you have full control of the server and reverseproxy - real dedicated CODE installation is easier to configure and maintain.. but nevertheless built-in CODE is expected to run at the same URL and NC ships it’s own proxy to access it:
BTW:
review the guide and you will see the 
is not even half of the working setup.