Thank you for your comment and sorry for the delay,
your question is exactly the point where I run short of scientific explanations, indeed. I place myself from a practical point of view. It is true that I preferred being cautious when I gave advice to plug only 3 x 8 ohm speakers in parallel on an amp output that can stand 2 ohm.
Depending on the amp quality, the scientific rigour of the measurement of the impedance of the speakers and the conditions of use, plugging more than 3 is running a risk of an amp failure because of heat. But to be honest, I did use 2 x 4 ohm in parallel on a 2 ohm rated amp output, without any problem for the time of a concert. Nevertheless, prudence is a good guide.
With impedance, everything is about a ratio between the output and the input, I believe that depending on the quality of the amp, the security margin can vary between set-ups, even though they have the same ratings.