Hello semper1,
Manipulation of settings can be done in the GUI or it can be done on the command line. There is a command to turn the compression to the desired settings. Manual manipulation of the PLIST file is not something we anticipate or support. I understand that for iOS there is no command line you can use. So I'm sorry I can't provide a complete answer for you here.
See here for command line parameters:
https://openvpn.net/vpn-server-resources/command-line-functionality-for-openvpn-connect/
As a sidenote, you should look into turning compression off. It is vulnerable to the Voracle attack. See also this security advisory:
https://openvpn.net/security-advisory/the-voracle-attack-vulnerability/
Kind regards,
Johan