Oh, I see, it's one of those convoluted Debian configurations. I'm not saying "don't use Debian," but I think sometimes they make things much more complex than they need to be.
Hopefully some other Debian user will come along with a definitive answer. However, if you want to go exploring, my method should allow your server to handle the hostnames you want, without any redirects. Nginx configuration is pretty straightforward--if you find the main config file (I checked the Debian package DB, and there is an 'nginx.conf', so that should be it), it should contain one or more 'include' directives; if you follow those to whatever file contains the 'server_name' line, you should be able to make the change there. Best of luck with it!