If you want your signal separated from the noise by 3 sigma, then you can make this cut safely only if your lowest signal is 10 pe or above. At present, there is a fear that S20 will have more dark current than bialkali. This is true in theory, but is rarely borne out in practice. I don't know their noise characteristics. This will be worse in the multiplexed case.
Multiplexing makes the tolerances a fraction of a mm.
MC results should not cut off at 1 pe. This removes a factor of 3 from the Philips dynamic range constraint and probably also removes some of the multiplex problems. Front end amplifier may be more expensive. Also, the falling edge threshold is more sensitive to baseline fluctuations due its happening in the flatter part of the exponential. Or at least study the problem and come up with a concept.