I am beginning to believe the clipping is NOT attributed to its DRE scheme
I see errors in your reasoning:
1. “JM20 Max H mode is turned off” - ok. I don't see any reason to doubt it, but it doesn't show a direct correlation. Here is the proof. I have two dongles with bare CS43131 and CS43198. Both exhibit clipping at frequencies below 8Hz and inaudible clicks. The first one shows signs of a working power-saving mode: when the signal drops to -12 dBFS, the current consumption drops immediately, and then after about 5-6 seconds it drops some more. The second dongle shows nothing of the kind.
2, 3. Judging by the strange behavior of your jm20 max, DRE is present in it, but not working. At least it doesn't work correctly. In stepped sine measurements the signal rose and fell linearly, so the response to more complex signal behavior is unknown. If you look at the “backward” curve, up to -50 dBFS the DRE does not work. If you measure the DR under these conditions, you get no more than 121 dBA instead of 127(?) dBA.
I think this is confirmation that the DRE here is not working correctly, it's likely that something is interfering with it.
That the CS431** are monitoring the signal is beyond doubt.
It turns out that REW (or at least the beta version I used) does not correctly plot the IMD sweep results
Indeed. Release V5.31.3, July 25th, 2024 works correctly. imho it is better to redo graphs with such errors (I saw your warning).
Because I used BW 48 kHz for this measurement to exclude the ultrasonic noise shaping's effect. This is just like what Amir does when a DAC exhibits this issue. Increasing the BW will make the results dominated just by the ultrasonic noise level.
That's the point, these noises affect the sound quality, and ignoring them reduces the objectivity of the measurements. A dense noise hump degrades HF interference even worse than normal harmonic distortion. Look at the effect of ADC (and DAC) Fs when measuring cs43131 SMPTE:
Fs=44.1 kHz IMD SMPTE = -107.3 dB

Fs=96 kHz IMD SMPTE = -101.6 dB

Fs=192 kHz IMD SMPTE = -99.6 dB

Fs=384 kHz IMD SMPTE = -96.9 dB

The filter settings of 20...20000 Hz are the same everywhere, but the smpte degrades by about 10 dB due to the influence of noise entering the digitization bandwidth.
Also look at CS43198 THD+N 10 kHz. The test signal is 24/44100, digitized at 768 kHz. My ADC also has a noise hump at 364 kHz, but it only goes up to -120 dB in this mode at 512k FFT.
I understood that the jm20 max suppresses the DRE for some unknown reason, the behavior below 8Hz is different and getting clipping/clicks to appear is problematic or impossible. Ok. Unlike ultrasonic noise, I don't consider neither clicking nor DRE working or not working as a big problem. Just showing what I suggested you measure obtained for the CS43198. The right side should be similar to jm20 max. The THD calculation was done using the 9 lowest harmonics, signal level -20 dBFS. Notice that the THD results are worse in the audible range because of the low signal level.