1) You can not use net-speed.com for email addresses now on that server. If you plan to use this server as emailserver as well for domain net-speed.com, you will have to reconfigure the hostname in /etc/hostname, /etc/hosts, /etc/mailname and postfix main.cf file.
2) Thats ok. You add all IP addresses there that you want to use on your server. But dont forget to add the IP in the network card settings of your linux distribution too as described in the manual.
3) See pages 87, 241 and 245 of the manual. You select the IP address where the domain points to in dns or *, if its not a ssl website and no other website is assigned to that IP already. If you select the IP address as shown in chapter 5.3, then you are on the safe side.