** Update **
Now on build R57, and this .capx stills either shuts down with no message or throws the JIT debugger, when trying to save.
All other .capx (demos, downloads and self-tests), and a newly started breakout .capx are working perfectly.
I think I'm giving up on this as an 'X-File' and moving on.
If your vision so exceeds your ability, then look to something closer.