USB is practically always better than I2S, unless the USB is somehow faulty.
This is explained in the manual. M is always million taps. Mx is like other filters where number of taps is relative to the conversion ratio so the filter response stays the same. Mx is million taps at 1:16 ratio and two million taps at 1:32 ratio.
Because Holo Audio R2R ladder is linear to 20 bits. So we utilize high rates, noise-shaper and just the linear range of DAC to gain lower noise and wider distortion free range. Much more output precision than you would get at 24-bit.
Input device doesn’t have settings for such. So input device doesn’t matter in this context.