The way we handle UI parameters is also not optimal; they are mostly hardcoded in the question edit form. So there is a series of checkboxes (with corresponding columns in the database table), and the PHP code decides based on those which UI parameters to send. We have been thinking about supporting other UI plugins as well, but we would have to generalize our setup in that case.
It's indeed unfortunate that we had to fork, but as you also assessed in your previous message, the custom question editor UI we needed would require many changes to bring it to CodeRunner upstream. Subplugins sound like a possible solution; it's a shame that they are not supported for question types (to be fair, we are not expert Moodle developers, so we were not even aware that this functionality existed).
Kind regards, Arthur and Willem
PS: No problem :)