I provide a lot of context for my questions. For the bottom-line-up-front, please skip down to the specific questions section.
My car was parked in a 3rd level sub-basement, which was flooded in Jan/Feb 2018 time frame. The water level was about 2-3 inches above the floor. A lot of good information and advice came from this forum.
The electrical systems on the floor and the carpet was replaced by a Honda service centre. They did not, however, check the hand brake line, which often froze after the flooding. While it was among the first problems that I mentioned, there were enough complications during the rather broad remediation effort that I find it understandable for the brakes to have fallen between the cracks. Since the remediation was completed in the spring, however, I did not experience the freezing brakes again until about a month ago, when the weather started to fluctuate above and below freezing.
Over this period, I also had two occasions to try using the defrost, and it didn't seem to be functioning. Not only did the rear window and side mirrors not defrost, but I could not feel any warmth from the defrost lines or on the side mirrors. Since The trunks lights had initially not been connected after the flood remediation, I assumed that something similar happened to the defrost.
Attempt to service
Due to my hectic schedule in the month of December , I knew that I would not be able to respond to last minute crises and bring the car in for servicing. I tried for two weeks to coordinate with Honda to lengthen an appointment that was previously scheduled Saturday afternoon in order to give proper attention to the brakes and defrost. That appointment was originally meant for a B-service package and an airbag recall. It was tire changing season, however, so no response was forthcoming. Only after escalating it did I get assurance from the service manager that the brakes and defrost would be properly handled within the same time frame.
The originally slated B-service and airbag recall were implemented. The defrost and hand brake servicing left much to be desired. I'm not sure what was done on the brakes, but apparently, nothing was amiss. I'm pretty sure that the cables weren't checked for trapped water. The defrost was "deemed" to be working because a test of some electronics module at the rear succeeded. I don't think that you have to be an electrical engineer to know that if a defroster that doesn't actually defrost, then deeming anything doesn't change that fact. When I asked to pose some questions to the performing technician about the module test, I was told that he had left for the day.
The specific questions
(1a) How often do defrost problems exhibit themselves despite the successful passing of a test of the unnamed module at the rear?
(1b) What might be the cause?
(2) Can one feel the warmth of defrost lines or the defrost in the side mirrors if they are working? I did not feel anything on the rear defrost lines, nor on the mirrors.
(3) Speculatively, what kind of checking could possibly have been done with the brakes to conclude that nothing was amiss, even though the brake lines freeze up?
 Extensive work commitments, medical appointments, and out-of-town family commitments during the holidays