Moving away from this particular case - are there any system failures that can result in less than atmospheric MP other than an induction blockage?
To see the MP reduce, I believe you must either have an increased intake restriction (e.g. throttle closure, intake blockage/icing etc.) or increased engine RPM (effectively causing increased suction from the engine and hence increased pressure difference across any existing intake restriction, intentional or otherwise).
So I can see how a failure of a CSU would show up as reduced MP as well as increased RPM. But a failure that just results in decreasing MP (and no RPM change) can't, to my mind, be caused by anything other than an intake restriction of some kind.
Mark