Just in short:
Collabora/Nextcloud integration:
we are on Linux here, APIs are like dust in the wind (sometimes). So if there is a magic way that the integration works no matter which version and API, then I would like to read that in some documentation, probably at the very beginning 
Host aliasing:
Yes I read the doc you are pointing to, it explains exactly what I would have expected. But unfortunately the inverse case of a single host and multiple aliases Is not mentioned in detail in the form that this does really work. It’s only implicit in the way that 2 hosts with separate aliases work side by side, meaning not collaborating.
My config is exactly as the example says, only with one host.
I am not talking about a loadbalancing collabora at this point.
Just one simple server.