1. Rode on the river paths more times than I can count. And yelling out "ON YOUR LEFT" is definitely good and expected etiquette. Just yell out
Sacrificing a few breathes and your pace is not a good excuse.
This is a public path, not the Tour de France.

2. I usually try to pass fast, but mainly so I can discourage the other rider JK.

3. If you're sporting headphones, just use your eyes more. Which basically only means to look behind before passing.