Many interesting thoughts here.
My .02:
Regarding the focal length of the eye, I think it is only important in relation with the size of the retina because both factors determine the field of vision, of which only a tiny central zone is sharp. As the retina is much smaller than a 35mm film, the actual angle of sharp vision is the same as a telephoto lens on a 135 camera, although the focal length of the eye is only 22mm.
As for the "ideal portrait focal length," there is none. There is only the 1.5m rule: if one wants to shoot any subject without the exaggerated perspective effect improperly called "distortion," then the distance between the subject and the film needs to be at least 1.5m. If the subject is 1.5m or more from the camera, it will look OK on the picture, even with a wide angle. If the camera is only 0.7m from the subject, the perspective will not look right, even with a tele lens. If you enlarge the image of a person's head shot at 1.5m with a 21mm lens and compare it with an image of the same subject taken with a 90mm at the same distance, you will find out both pictures to be identical, except for the grain and lack of sharpness due to the bigger amount of enlarging of the one taken with the wide angle.
So, in theory, if you like your portraits tight, use a 50mm; if you prefer environmental portraits, use a 35mm. But of course nobody forbids you from using a 21mm to shoot people at 0.5m 🙄
Cheers,
Abbazz