Correct. Good point -- I'd forgotten this aspect of light.
Correct. Furthermore, Magic Resistance wouldn't affect who is illuminated.
Medieval scholars would disagree that lumen is a medium; rather it is a quality imparted to a transparent medium (air, water) by the adjacent lux. But that's just nitpicking 
As already pointed out, it is lumen that does this, not lux. But in making this mistake you are in common with several medieval scholars who often confused the two words. This is at least partially attributable to much of this work being done by Muslim scholars, whose texts were translated from Arabic. It appears that the translators were not consistent in their use of terms.
Agreed. They cannot work through normal vision. Cats and other animals that can see in the dark emit rays of light from their eyes, enabling them to see in the absence of ambient light. The proof of this is that you can sometimes see the eyes of cats glowing at night.
(this was part of a mostly-discredited optical theory by the C13 called 'emission' or 'extramission'; however, not all parts of that theory had disappeared. This is also how the basilisk kills, and the evil eye is placed)
Lux is only a quality inherent in a light-producing object. Lux doesn't radiate, any more than my size or weight radiates (since size and weight are qualities inherent in me). You can fail to create an object with the quality of lux inside Magic Resistance (e.g. using CrIg to make a light in my hand), but that's a different matter. No one is trying to argue that.
Lumen is a quality imparted to transparant matter by an adjacent lux-producing object, that allows that matter to generate and propagate visual species. If I'm standing next to a magical light source, the air about me can now generate species, so other people can see me. Unless you feel that that magical lux should cause the inheritance of magical lumen, Magic Resistance has no effect on illumination. Personally, I feel this latter position is untenable through analogy. Magically giving something a blue coloration does not make the medium transmitting the blue species magical --- people with Magic Resistance can still see the blue colour. So magical brightness does not cause magical illumination.
(That's not to say that magical illumination is not possible. A Muto spell could grant air the quality of lumen without a source of lux. A dark room could be lit up with no light source, and those with Parma Magica within the room would not be illuminated because the air is now a magical medium and is stopped by their Magic Resistance. All individuals (including those with MR) could still see any object or being that is illuminated, because the species generated would be non-magical despite being initiated through magical means. It would be a great way for a cabal of Tyatus magi to meet!)
Mark
Best wishes,
Mark
Nice! With this second dose I think I will digest correctly now.
Those 2 posts were so helpful, I thank you from the bottom of my heart.