• Athiril's math indeed is just a description of the characteristic curve (exposure vs. density).

mts' HLS/CMY math is a mistake. This extra complication makes it difficult to see if it is in error or not. But in fact, the actual error in his deduction is that the printing stage, which is simply a subtraction operation from the CMY values, is ignored. When it is added, we end up with exactly the same CMY and HLS values given that the H-D curve is straight. In the end, it is just very simple; the curve shape tells us everything. If we have a linear system, the bias point does not matter; we can make additions and subtractions as long as we are on the linear region. Exactly the same works in analog electronics.

If the curve bows or shoulders, the contrast indeed is lower. Today's films, however, have very long linear portion.

The old wisdom says that increasing exposure reduces contrast and thus perceived saturation. This has been a well-known fact. Somehow it got swapped around and became an internet legend in a completely opposite form.

However, with today's films, in most cases, there should be no difference in contrast and saturation with slight overexposures (+1 to +2 stops) due to shouldering.

I think it depends on what you are shooting. Athiril's speaking about sunrise and sunset, where +15 stops is well possible when metered from the sun and nearby clouds compared to the landscape. This can really land on the shoulder of the film, unless metered like it was a chrome film - from the highlights, then adding a few stops. However, with neg film, you can of course place them higher than with chrome film (e.g. +5 instead of +2), but there still is a limit. If you measure a sunset scenery "from the shadows" as usually instructed for neg film, you go over this limit. "Expose for shadows" is just a rule of thumb for most subjects.

OTOH, even if there is no shouldering, there still is the toe. With subjects with important shadows with colorful objects in the shadows, by overexposing a stop or two, you make sure that the shadows are not on the toe - and increase their saturation and contrast, without altering the saturation and contrast in midtones and highlights - given that the highlights are not difficultly high. This part is true in the internet legend, but the legend is wrong in making this a rule of thumb which it is not. You need an understanding of shadow contrast vs. midtone contrast vs. highlight contrast and understand how this relates to a particular subject.