I'm the OP, and it's now been 6 or 7 weeks since I started this thread as a total newb. I've now put about 700 miles on the SART, and I figured I'd share the 'answers' I settled on.

1) I yell 'on your left' to anybody riding near the center line, or any groups of riders 2 and up. I also generally get pretty close behind them, slow down, and then announce myself. When I encounter a solo guy riding or walking in his lane, I'll simply go by them at full speed without announcement as far to the left as I can get.

2) I hardly get passed anymore so this question is no longer relevant. The road bikers that do pass me generally accelerate to pass, then settle back into their normal pace after they pass.

3) Headphones: I wear them about 10% of the time now. However, when I do have them on, I still seem to hear everything from birds to road/chain noise, so I don't think it really affects hearing any more than having the windows up on a car. If I'm on a public road, I take them out of course.

The most annoying things I've found on the SART:
1) Rollerbladers that take up both lanes with wide swoops, while wearing headphones cranked up so loud they can't even hear me yelling at them from 5 feet behind them.
2) those 4 wheeler bike things near yorba linda park driven by teens at 3 mph on the wrong side of the road.
3) the fratboys wearing their abercrombie shirts riding around on their beach bikes tossing beer cans on the trail edge. I gave one a very sarcastic lecture on finding trash cans, and he did seem at least a little embarassed to have been caught trashing the trail.