2a) That’s correct, you’ll always need the physical interface to actually transmit the data.
2b) The normal gateway will remain the default gateway. However, some extra entries might appear in the routin table of the computer.
If you want to play with VPNs on your computer then you could use two virtual machines with OpenVPN (one server, one client) or you could try GNS3 to emulate a Cisco router and use one virtual machine as the VPN client.
Normally with a VPN client, the VPN server won’t know the network behind the computer. Also, your computer is no router so it won’t normally route packets from one interface to the other. If you would have a site-to-site VPN then you need to manually configure which networks are behind each router or you need to push them.