The single-photon assertion is a bit controversial as it depends critically on the angle at which the light is seen (b/c of the distribution of rods on the retina) and on the condition of the eye and how many carrots the subject has in his/her diet Turns out that the brain filters out much of the noise containing the single-photon information. But, in the statistical sense, the scotopic vision is actually sensitive to a single photon.On a clear dark night the eye can see a lit candle from 30 miles away
Regarding light under the doors, of course the very safest policy is to strive for complete darkness, but I am reporting, with very high confidence, that a bit of light under the door in my darkroom has had no measurable effect. And I have worked with IR film extensively.
There are many things in photography that people sweat and fuss over but which ultimately turn out to be moot most of the time. This is not the right profession for worry-warts: their creativity can simply grind to a miserable halt. There are just too many potential technical pitfalls to obsess over: MLU, 1/FL, aperture for best MTF, center post or not, base fog... etc. etc. Best policy is to experiment, establish your own best practices, and go with what works for you.