Thanks for your thoughts on this one...
Crossunder. Unfortunately I get the definite feeling you are the one mixing two things up, "apples and pears" as you put it.
There is that sort of frost you get from cold-soaked fuel, what the Boeing file refers to. You may see that forming on a parked aircraft at temperatures above freezing and I am sure Boeing are on firm ground in saying it is of little import.
There is another sort of frost that forms in temperatures below freezing that covers the whole aircraft, not just the patches of wing over the fuel cells. As you have so emphatically pointed out that MAY be of no import.
On the other hand, large accumulations of frost definitely have been known to cause crashes. There was a recent high-profile crash in Russia where a large business aircraft lifted off, immediately rolled into a steep bank and crashed, killing all onboard. The cause was taken to be wing contamination. This is the reason for the FAR I loosely cited: not being able to find a clear boundary between what is safe to fly with and what is deadly.
You, coming from Norway, may well know a lot more about flying with frost or ice than I do, given that I, coming from Africa, know very little. Does this superior knowledge of yours also mean that you would be happy to violate an FAR? That seems pretty sporty to me! Here you seem to be telling our readers that you really do not feel compelled to follow a rule that you somehow know is irrelevant, somehow being able to just tell what is safe and what is not from experience. I don't want to cause you any sleepless nights but it might be so that the FAR exists because no one can do that!
Okay, I have seen my aircraft all fuzzy with frost, when it was pretty clear even to this winter novice that no one was going anywhere until we got de-iced. And on the other hand I, too, have seen small patches of frost that I must have somehow forgotten to notice, when the aircraft didn't seem to mind one little bit. The problem is, "Where do we draw that line?" Well, we don't have to, do we? That is precisely why the FAA made that rule, I guess.