Good news: The plumber was back today, and in addition to the new relief valve and new shut-off valves installed on Thursday, our water heater now has a new expansion tank on the inlet and new inlet and outlet connections (both of which were leaking). Hopefully this will be the end of our water heater problems.
Bad news: I've just had to call the podiatrist and cancel my followup appointment for today on 20 minutes' notice, to be rescheduled later when their computer system is back up, because the car won't start. I'm still not 100% certain whether it's a weak battery or a failing starter; both could cause the observed symptoms. The least expensive of the probable fixes is to replace the battery, and it's probably due (it's five years old), so we'll try that first. With luck, I can get it done today, if I can get the car over to the battery shop across the street. I don't know if I can manage to do it myself or not; while I'm perfectly competent to do it, the battery is pretty well buried down in there, and I don't know if I can bend over the engine compartment long enough to do it with my knees and back in the shape they're in.