So you have to convert it back to voltage to compare it to the voltage sigma that is measured from the read out. Running at room temperature, this should be pretty close to true. With all these pixels one can just put a box between cosmic rays and measure sigma. At room temperature the noise in the system swamps the cosmic rays and unless I put a small box around a cosmic ray, I can't tell the difference. However, your interpretation of the triple peaked distribution sounds right.
Stuck bits are equally likely.
This is the differential linearity specification, and successive approximation ADCs are not all that good. But they are much slower. The problem I have is a standard one. I came in to Fermilab the next day after I understood what the problem was and the first person I discussed it with had had a similar problem. It can take a while to fix.