Another thing is that my openvpn server is running as : --config openvpn-vpnserver.conf, where conf is store in /tmp/etc/openvpn-vpnserver.conf , why in /tmp/etc ??? pretty mess.
It seems you are using some previous guide version which utilizes UCI configuration.
It's designed to operate this way by OpenWrt developers to separate runtime configuration from persistent.
The current version of the guide uses a simpler approach.
Both methods have their pros and cons.