For the first, physical hues form a straight line continuum, from red to violet. The human eye samples these hues at three overlapping points (RGB) and then the visual pathway does some math to reinterpret RGB as a closed circle of hues we see in our visual field.
For the second point, while monospectral light cannot produce magenta, you don't need multiple molecules to reflect multispectral light.
>the visual pathway does some math to reinterpret RGB as a closed circle of hues we see in our visual field<
sort of, computation of 'contrast' so to speak, or, in broad terms, differences in wavelengths
most likely done, foremost and in essence, in area V4, cortically. if damaged, cerebral achromatopsia [colour blindness results]. patients won't even see/perceive induced 'coloured' visual phenomena, such as phosphenes [Farady made a few quid by electro-shocking ppl, for instance in Jardin des Tuileries, Paris : electro-magnetc impulses applied transcranially to the occipital part of the brain can elicit, in some subjects, brief bright colourfull light sensations]
For the second point, while monospectral light cannot produce magenta, you don't need multiple molecules to reflect multispectral light.