Actually in case #1 you probably wouldn't have a problem. When a road warrior connects, and talks on the VPN tunnel, the traffic from the client should be coming from its OpenVPN client IP, not the IP it obtained from the coffee shop network.
In case #2 you would have a problem trying to reach anything in that subnet, yes. It would believe it was local. You could setup some 1:1 NAT for another unused subnet that people can use in that case though, like 172.20.11.0/24 that maps on the OpenVPN interface to 172.20.10.0/24 on the inside. Then if you have a conflict, the clients just connect to IPs in the alternate subnet.
Though with that odd of a subnet I doubt you'd ever hit a coffee shop or hotel using that.