VPNs make services tricky because they edit the routing table. They redirect all TCP/IP traffic through them, except for traffic to the VPN server, which includes re-routing what would have been a LAN IP address. It sounds like something similar might be happening.
You can have a look at the routing table using the ip route show command. If what's happening ...
The normal way to access devices behind a firewall from outside is to configure port forwarding on the enterprise router/firewall. So you have to ask your security manager if he would like to give you access to your RasPi behind the firewall. There are of course ways to peek holes into the firewall but I don't believe that your security manager does like it.
I would say the easiest solution for this setup is to use a bridge because there is already a DHCP server running so we don't have to worry about it. You tagged systemd-networkd so I will use it. First switch over to it with:
# disable classic networking
rpi ~$ sudo -Es
rpi ~# systemctl mask networking.service dhcpcd.service
rpi ~# mv /etc/network/...
The message Address already in use means that there is already another process that has bound that address to a socket. Most of the time it is another instance of the same program, in this case another openvpn process.
Use sudo ss -ntuap | grep :1194 to find processes using port 1194.
If the address 10.10.10.10 from the first line of the output is the ...