More juggling acts... pending a better solution.
Short story though... I don't know of anything that can directly get the job done.
To return to the topic at hand for a moment... that of deleting a project.
In a sense that is very simple.... just delete the project directory and the project is gone.
However, this hints at a way to delete drawings no longer used as well... although by taking a slightly longer route (that just happens to clean up the Project files structure in the process.
Via the Browser Room if/when we Right Click on a scene file there is an option to 'Collect Assets'.
Do this should (at least in theory).... collect all the assets related to the Project (I believe I've tested this before and it fell short of my expectations).
But... assuming that works or can be made to work and collect all the Project assets...
Now we want to use Export Scene and then (importantly) specify a new Project name and location.
This should... again... in theory... create a prestine Project directory with only the assets associated with that Project and scene.
It is THIS Project we want to keep and use.
The other project we can now abandon... thereby 'removing' all unused levels because they were never brought forward into the updated Project.
This also (in theory) could resolve the issue of sharing projects because this new prestine Project directory is ready to be zipped up and shared with others.
Yes, it needs a little tweaking, file integrity validation and automation but it has potential.