And is the gauge likely to tell you anything useful if you have an engine failure? Many would expect it to show a reduced MP.
That's an easy one. If the engine is normally aspirated the MP will be below ambient pressure when the engine is running (or even being cranked by a windmilling prop). It will obviously revert to ambient once the engine has stopped. If the engine is turbo supercharged then it will drop if the MP was above ambient (toga or climb power usually).