1) No, you dont need virtual IP addresses to run multiple websites. One IP is sufficient if you dont need SSL for every website.
2) This are two different things. You must setup the site with your internal IP in the ISP-Manager and then add a primary DNS-record with your external IP in the DNS-Manager.
3) Do the same as for the first site.
When you setup the A and MX-Records at Namechep.com, you dont have to use the ISPConfig DNS-Manager at all.