The content filter runs out of the box perfectly if the system is installed as described in the perfect setup guide. I'am doing these installations daily and there are > 10 000 installations with ISPConfig out there were it works. So instead of claiming that it does not work gernerally while it just not worked for you, why did you not just asked if you had a problem?
to 3) I work also in a software company and it never blocked any zip file in the last year since we use it, so the question is which settings you use. Amavisd is a widely used spamfilter.
to 4) That was your decision. ISPConfig is a multi server system to separate mail and webserver. If you decide to install web and mailserver on one machine, dont blame the software for it.