So - what should I have done? Easy. I really should have cut each laminate separately instead of gluing 4 layers together. That way I could have made the airway near the mouthpiece much narrower, and have achieved a much more gradually growing airway shape. I think this would vastly help the backpressure problem.
Mainly, I also should have spent more money and made this out of plywood. MDF was a mistake - and I would urge you to not go anywhere near the stuff if you plan on doing anything involving breathing near it. Seriously, this stuff also has glues in it, which are probably not good for you either. I really, really should have thought this through more, before setting out.
But hey! That’s what experimenting, and making prototypes is all about. I made something, which is very nearly perfect, and I am determined to make one which really will work.