Not selecting apache2 AND nginx useflags is not possible. That should be allowed in order to use an alternative HTTP server without pulling dependency of apache2 or nginx.
We can add more use flags, but we need to set ownership of files unfortunately. /usr/share/${PN}/bin/icingacli setup config webserver nginx --document-root /usr/share/${PN}/public I'd be fine with a patch, but I don't see a way around that right now.