Proofing scenarios adds extra complications, particularly if the proof-reader hasn't actually played the scenario. It's frighteningly easy to miss a setup problem. I like to use VASL to set up scenarios when I proof them, just as if I were setting it up to play. Unfortunately, in this case, that wouldn't have been possible (I should say I was not a proof-reader on this product). Second-best option is to physically set it up, but that may not have been an option either (the scenario proof-readers may not have had any access to the new boards). So if you're just "visualising" the scenario in your head, it's very easy to not notice issues like that.
There's no simple solution.
You raise an important point. One that hadn't occurred to me before. Namely that proofers don't necessarily have access to the maps/overlays used in a particular scenario design. (Proofing overlay coordinates is tedious enough when one has access to the boards and overlays.)
Board configurations (and to a lesser degree overlay placement) together with set-up/entry instructions are a common form of scenario errata. They may well be the most common type. Unlike errata that addresses scenario balance, the parameters of the playing area are (usually) fixed. Get them right the first time and the primary need for errata would be confined largely to issues of scenario balance.
An example of the importance of proofreading is
ASL Annual '95. Of the 24 scenarios in the magazine, at least eleven had errata issued shortly after publication. Five instances were directly related to the playing area. One dealt with the direction of entry, two had boards that had been rotated 180 degrees in the wrong direction, one failed to indicate what hexrows were in play, and a fifth scenario had the north arrow pointing south. (Errata for A73 addressed the fact that the sides setting up and moving first were reversed. Scenario A82 was missing three SW mortars. Only one scenario had errata for the VC. A couple fiddled with the balance provisions.)
I agree with Eagle4ty that playtesters can help catch map/set-up errors. However, playtesters generally don't have access to the proofs, working instead with draft scenario cards. Therefore, more could be done to help proofers spot oversights. Perhaps, in MMP's case, the project lead could make the boards/overlays (or a png of the playing area) available to proofers.