Is it out of gamut if it is reproduced oversaturated?
My undestanding is that "out of gamut" is when a device can't reproduce/capture a particular range of tones (wavelenghts). From the plot above, the reference point for Red would mean the 'ideal' amount of tones... so MKII falls behind it.
I'm not really a color specialist though, so everything I say about this topic is probably innacurate in some way or another.
I once read somewhere that input devices like a camera don't have a gamut, and I still like to think that way.
It's just a bunch of photodiodes, but when you try to classify that signal inside a gamut for real world usage, this is when things get complicated. This is also why (IIRC) some color spaces have "
imaginary colors".
If you find me some spectral data for reflectance of skin I can calculate how it will look on all of these cameras 
Good point. But the low Red sensitivity will affect these tones on MKII. I'm not sure if non-saturated reds would have a big difference, though.