Quote:

Do any of these data points help?


yes.

I think we can all agree the bulbs are not failing (:

http://gkb.servehttp.com/images/achevia/sch2.gif
as this image shows, both positive and ground are switched.
it would really be a good idea to find out which goes missing.
that's easy to do at the bulb.

does that car have Day Running Lights?
when the lows fail, what happens when you switch the main switch off?

seems like there's a good chance the breaker is just doing it's thing, maybe it's too sensitive, or maybe it's functioning as it should, protecting the circuit. but before you even bark up that tree, you should verify that power goes missing at the bulb on the hot side, if that's the case check to see if power is going in and out of the breaker. seems like you could just bypass it with a 20 amp fuse.

I would expect slightly lower voltage readings from your test, but if it's really cold and the test was done soon after starting the engine, it's not that far off.