The one thing I can answer is that multi-patterning does not shine light through two masks simultaneously. Instead, it consists of multiple separate steps.

I think for the rest, the point is that light arriving on the waver is not a binary thing, but due to refraction and self-interference light arrives in variable intensities. So within difficult constraints, this allows you to control the area in which the intensity is below our above certain thresholds. I assume that if you then manage to control the chemistry just right, you can then produce features that are smaller than the wavelength of the light -- under severe constraints of what shapes you can produce. You definitely do not get to produce an arbitrary bitmap of sub-wavelength pixel size.

I think I've listen to asianometry (YT channel) talk about this too, but I am unable to find any clip now where he explicitly talks about interference lithography...

then again, it's called the wave-lenght, not the wave-width

If you drag a baseball bat through sand, the edge of the cut "channel" is much sharper and narrower than the baseball bat.

Now offset the baseball bat a bit and draw another line which is partially overlapped over the first one. You will get the intersection of the two baseball bat wide channels, but it will be much narrower.