You are correct regarding the DRM. Who gets affected is a bit more complicated. Only those units that are hit and those units that are in the location where the counter lands (usually ground) take the MC. For instance if a unit is firing at a two level building with an enemy unit in each level where the ground level is hindered at +6, the first level is blocked (plateau-ed) due to an orchard and only the top level is clear, the unit firing could (only) hit the level two unit. If the top level were hit, the WP counter would be placed at ground level, and the units at the top level (which was hit) and at the ground level (which was not hit due to the +6 hindrance, but that's where the WP counter ended up) would take a casualty MC.
JR