You should be able to go advanced > add ( on master anyway )
Most of the options you mention are supported there at least.
Manually changing things is difficult, because you have to edit both the web files ( lua is hard but these are on the easier side ) and the init.d parser.... very difficult.
If an option is genuinely new / needed.... posting a feature request on the relevant ( github packages? https://github.com/openwrt/luci/issues ) site should be accommodated.
luci-app-openvpn is friendly in that way on the front end.... it's the backend and testing where odd options can be troublesome to implement without feedback from many.
I second the suggestion to use straight .ovpn files..... especially in cases like yours..... but notifying the developers is beneficial also.