>Parts lists show that the suppression position isn't always in the same location on 26 and 30.
This seems to be something applied by commuter lines according to their preference?

The 26-L brake system uses both the 26-C and the 30-CDW brake valves, depending on the purchaser. In a 26-C brake valve, the location of suppression is dictated by notches on a cam, and many RRs changed this cam so that suppression occurs before full service or handle off (has to do with controlling slack action when suppressing penalty applications). So you have to check on the date that part catalogue was issued.

WABCo part catalogue 3208-26 Sup. 2 (1964) for the 26-C type brake valve shows no handle positions, nor does G-g-5071-6 (1964), the 26 instruction manual. Locomotive operating manuals, on the other hand, do. Wabtec 4208-30, S.5 for the 30E-CDW (for passenger service with E-P brakes, 2003) shows handle positions, so presumably the manuals for other varieties of 30CDW do too.

>It should also be noted that in addition to a continuous brake pipe reduction (eventually to zero),

In techno speak, the handle off position drains the selector volume to zero, and this causes the high capacity relay valve to draw the train line down accordingly.

> the handle off position keeps the suppression valve in effect overriding a trailing or dead unit from causing issues trying to make a penalty application.

Penalty applications in trailing units are prevented by cutting out the trailing brake valves, which in turn prevents the high capacity relay valve from doing anything (neither charging nor reducing the train line).