I face many of the issues described by the OP and others on MUP I follow to work and back. I'm only ever on that trail when I'm on my commuter bike so I tend to go slower -- like 13 to 14 mph on a good day. On a few occasions I've had folks on road bikes come whizzing past me without me knowing they were approaching. It's frightening and infuriating when you don't expect it. It's also just rude. Seriously, it's a MUP, if you're that effin' ernest about your riding get off the path and on the road where it's safer for everyone.
On the other hand when I'm on the MUP on my bike I pass far more pedestrians than cyclist pass me. I too have seen that look of suprise that comes up when you yell out "on your left" only to have the person stop and spin turn around with a dumb expression on their face -- whaaa?. However, I recently learned something interesting. My new commuter came with a bell. At first I didn't use the bell and kept calling out "on your left". I'd still get the occasional stop and spin. Then I started using the bell....lo and behold folks seem to know that it's a bike coming up behind them and naturally step aside. I always say "thank you" as I pass.