If it is connected to a life circuit some where, it needs to terminate in a box. It's possible it's some abandoned piece of wire (in which case it's just sloppy but OK), but if it runs to power (even a shut off breaker) it should have been properly terminated.

The mystery switch isn't a code issue, just indicative of leaving the job half done. As long as the smoke is not connected to a switched circuit, there's not particularly anything wrong with using lighting circuit/box to power it.

Thanks for the input. Further investigation showed that the light fixture is not wired through the smoke detector after all, but is properly wired to two switches (incl. mystery switch). Given that another light fixture in the house is wired to an outlet rather than a switch, it is only mildly reassuring that this turned out to be OK.

The mystery live wire turns out to be coming from another smoke detector. Seems they thought they would be wiring one more in the series and then they forgot or decided not to but never went back to remove and/or terminate the wire.

We will pull it (because it's too short to be of any use) and take the opportunity while the walls are open and holes already exist for the wiring to add one more smoke detector in a spot we think could use it (hallway just outside living room which has a wood burning fireplace).

I still want to know how mad I should be at the electrician.

I wouldn't call the same electrician back for future work, but by the sound it doesn't seem he wants you to call him back anyway. I'd say simply not calling him for future work is a perfectly even and amicable solution for both parties.

Your complaints are a miswired light that turned out not to be miswired at all, and a single wire that was likely an incomplete portion of a job that was terminated prematurely knowing that it was incomplete. If you stop someone before they are done, they are not going to be done.

"Slow" can only be relative to other jobs and we don't know how your house is constructed or the scope or complexity of the job. It may have been more difficult than you knew or the electrician originally expected, and thus may have been taking longer than others. The same is true about the extraneous holes in your plaster. Actually, I hate plaster with a passion for exactly this reason. If your house is old enough that you're dealing with plaster everywhere, I'd say with relative certainty that the job was not the easiest.

So, on your proposed scale, I think I would place this all the way down below the lowest level, and label it "things were abandoned, not missed, and as much of the job as was completed was completed effectively.