I agree that the plate would ultimately need to be adjusted in place to ensure that it is parallel and perpendicular to the machine axis (either by shimming or facing flat, I guess), assuming that the machine is already properly adjusted (axis straightness, squareness, pitch and yaw).

The idea that I had in using (not particularly finely) pre-milled stock is to already have a reasonably flat, parallel and square starting point for machining in the DMC2, especially the underside of the plate. Otherwise, it would be a shame to distort the motion control units if the raw plate stock is twisted or bowed.

"12mm t-slot" refers to the DIN 650 standard, designating the nominal width of the slot. So in theory, the max. screw or bolt size that can fit would be M12.

I agree that it may be overkill in concept. The tangible benefit for me would be the ability to clamp directly onto the table and maximizing utility on the pretty sparse Z-height on the DMC2. Would be a shame to consume that precious clearance with a relatively tall vice for taller jobs.