Well with main game getting abandoned tag time to release an updated stable version just so it shows some activity like 51.1.001A or something lol
Was it that you guys got busy or just were waiting to see what all was going to change before doing a lot of work to prevent extra work?
If it is the later of the two. I have a suggestion.
Don't rely so heavily on his code. Make use of the assets, text and story.
You can do so without making it compete with his even if you replaced renpy for another engine.
Just require the people to download his game to get the assets then use a script to remove from his game what isn't needed and replace it with your system. It's not that hard.
I can say that as a fact because I started a project for my own purposes to put a GOAL based AI system in to the game.
I figured I would use it as a test bed.
It works out fairly nice so far. Better performance from the engine, cleaner code, and the game itself runs better even beyond the renpy replacement. (in short if I took the goal based AI system and world dropped it in renpy it would perform better there as well.)
If you are wondering I'm not even using my C++ engine just the python version of it with SDL2. Pygame and renpy each add some overhead. Pygame uses SDL's surfaces but doesn't use the Texture that are hardware accelerated. That gives a 33% boost in performance easily.
The point being whatever changes he makes after that won't effect your code nearly as much if he adds a feature it is just something you may want to add to yours. But it will never force you to rewrite your mod again to work with his to the same extent.