Regarding the discussion of square waves vs sine waves into a mixer, is it not true that we have become conditioned, i.e. by block diagrams of entities that have an "X" in them, to think that mixers act as multipliers of 2 waveforms in the strict mathematical sense when this is not the case with real circuits? To the contrary, I thought that most practical mixer circuits actually perform periodic inversion of the RF input based on zero crossings of the LO input, in which case the IF output is the same regardless of the LO being square (with 50% duty) or sine. Of course, having less jitter in the zero crossings of a square wave in noise over that of a sine in noise is the difference. I don't have any simple mixers lying around to play with, but I suspect that the IF output of one with a simple sine RF input and sine LO is more than just 2 lines, at (f_RF+f_LO) and (f_RF-f_LO), in the frequency domain due to this action. All of the odd harmonics of the LO frequency are mixed in there, regardless of LO waveform, so9 long as the zero crossings of the waveform are equally spaced. With suitable filtering, the fundamental (f_RF+f_LO) or (f_RF-f_LO) is what we care about.
Jim