1) do i need apache/nginx enabled on a CWP server?
will webmail and the back end all work?
2) No option to activate apache for some domain names and not others?
1a) No, CWP doesn't require Apache/Nginx to work. The backend uses cwp-srv.
Webmail is a Redirect for Apache/Nginx though, so if you have them disabled, then webmail would stop working.
2a) You could theoretically have multiple webservers running listening on the same port, but specifically for certain IP's.
But that would be overly complicated, and not guaranteed to be 100% for production use.