@tet I’m not the person to answer the first part of your questions but I can help with the ribbon tweeter frequency range part as I have background in psychoacoustics research from my degree dissertation
the ear only able to detect up to 20khz, whilst a good general rule of thumb, is only partly true. Although 20khz is the roughly the highest frequency that’s audible to humans some studies show that much higher frequencies can still be detected by the inner ear even if not ‘heard’ in the traditional sense
these ultra sonic frequencies can still cause physiological reactions from the inner ear so despite not knowing we are hearing them they can add additional psychoacoustic enhancements to a music listening experience
a good example of this is listening to orchestral recordings vs a live orchestra performance. In the real world all instruments have resonances and harmonics both in and out of the audible hearing range, this goes into both infra sonic and ultra sonic frequencies. Particularly high string instruments like violins and metallic instruments like brass sections and percussion cymbals have many harmonics in the ultra sonic range that can be measured with calibrated spectrum analysers.
These add to the experience of hearing the music performed live in front of you that can be lacking if listening back to recordings where everything above 20khz is cut out of the recording and/or the speaker playback.
from an electroacoustic perspective there are also potential benefits to playing up to 30khz-35khz or even above.
The higher above the limit of human audible hearing a driver can play the cleaner with less distortion it should sound when producing 18khz-20khz. This can result in the same piece of music sounding more soft, airy and open specially on ribbon tweeter playing to >35khz than a silk dome that stops around 22khz for example.