3) Smaller pixels -- the direct product of higher pixel density -- generate more "noise."
6) ... Because by default, the 28mm lens from the same shooting position as the 38mm lens yields greater DOF at the sensor if both lenses are set to the same aperture.
you have some good points, but the two I quote are at best ambiguous, and are particualry dubious on yout rterms of comparison with equal pixel counts, whc I accept asa god way to compare.
As is so often done, you are silently assuming that the smaller sensor and smaller pixels will be used with a lens of not only smaller focal length but also of smaller aperture size (aperture diameter), in proportion to the pixel and sensor size. This assumption leads to the need to use the same high ISO speed setting when a given high shutter speed is needed. Thus the smaller pixels and format are asssumed to use a probably smaller, lighter, cheaper lens, leading to more DOF and less light gathered to each photosite, lwoering S/N ratio.
Of course, using a smaller, lighter, cheaper lens with a smaller aperture diameter has exactly the same effects even if you use the same size pixels and sensors, so I do not know why people keep blaming this on the sensors or pixels instead of the uneven choice of lenses hidden in such comparisons.
At most, sometimes a smaller format and focal length limits one to a smaller aperture size, and then one is truly limited to lower speeds, higher noise and/or more DOF along with likely weight and cost savings.
But in the telephoto regime at least, size, weight, cost, and DOF needs tend to set roughly the same upper limits on aperture size, regardless of format. For example, if one's limit is f/2.8 at 200mm, it is probably f/4 at 300mm and f/5.6 at 400mm. So if instead the smaller format or smaller pixels can be used with a lens of equally large aperture (diameter), leading to similar size, weight and cost, it will get
1) aperture ratio lower in proportion to focal length,
2) equal DOF (on equal sized prints)
3) lower ISO needed to get the same high shhutter speed,
proportional to the area of the sensor and pixels
with the lower ISO setting in turn leading to
4) equal amount of light gathered per pixel and thus equal "signal",
measured in electron counts.
Adding the fact that
5) smaller photosites produce if anything slightly LESS
electrons of thermal noise, particularly in long exposures
we see that smaller pixels will then give about equal signal to noise ratio, and might be better in long exposures.
P. S. In a website with a leaning towards landscape photography, it puzzles me why so many people ignore or even actively deny how common it is to have to stop down for adequate DOF, at least when asserting the inferiority of smaller formats.