<<< I have managed to restore internet connectivity on one machine. I discovered the router was communicating with the internet via a different default gateway and DNS >>>
These addresses are allocated dynamically by the ISP to the router and a change in them wouldn't normally have any effect on your "Local Area Network". However, if you have manually assigned the ISP's DNS addresses in your LAN settings (instead of the more usual setting of the router's IP address as the DNS server) then this change will affect your internet access. Is there a particular need to be manually configuring your LAN addresses?
<<< (1) Why is the subnet mask of the WAN 255.0.0.0? Is this significant. >>>
The subnet mask used by the ISP isn't something that you have any control over or something that affects your local network. It's allocated by DHCP along with the IP address, so just ignore it.
<<< (2)How do I restore the home network? (3)Does it matter what IP address each machine of my home network has - 192.168.2.x >>>
The problems with your home network may be due to the IP addresses currently used, or possibly your firewall if the addresses have altered and the new addresses aren't allowed for in the "trusted area" settings. The number in the final octet of the IP address can be anything from 2 to 254 (as the router already uses "1").