1st issue: I created a LAN to LAN connection and it disables both ISP WAN internet connection.
1Q: Why is that and how do I re-establish ISP WAN on one or both Zeroshell routers?
Please, post me further details about your configuration. Probably you have assigned to the vpn interfaces wrong IPs
2Q: Do both server & client Zeroshell router need to have a static IP address on its own VPN LAN to LAN interface?
No, they don’t need to have a static IP. In any case I do not understand the reason to assign them by dhcp
3Q: Do I need to setup a Static Route?
If you use a routing configuration, you need static routes. In bridge mode, where the layer 2 of the LANs are bridged, you don’t.
4Q: Once the Zeroshell VPN LAN to LAN connection is configured correctly, can each router continue to user its own ISP WAN internet connection for internet browsing and only use the VPN LAN to LAN for File and Printer sharing?
yes, it is possible. You just have to add the static routes to define the traffic you want to be tunneled in the VPN.