a) Real world scenes often go below the Nyquist limit ( e.g pattern from a brick wall at sufficient distance)
b)In a camera without a low-pass filter (e.g. the Ricoh GR) in practice you don't see Moire but you can definitely generate it artificially off real world objects
c) The Point Spread function seems quite important (e.g a 1-pixel scene feature for the GR will have half the effect on the sensor as a 2-pixel function because of the point spread)
d) I'm a bit confused as why Fourier/ DCT all work as well as they do, as the Nyquist assumption for a modern digital camera seems incorrect, but on the whole sub-pixel matching in a Fourier space (while Nyquist assumption is not true) seems roughly on par with matching in the spatial domain - I would love to see something that explains why this is so
Happy to discuss this more as I am still learning