The mag check should have proved mags were functioning correctly, including the RPM recovery on reselecting both. Therefore if MAP is at 20" when rotors are running on the ground, prior to applying pitch, something is wrong, MAP should be around 13" -15" and I have taught this as part of pre takeoff checks, since that tragic accident at Sandtoft when a solo student had only one mag selected. This check is the best way to spot a dodgy/unselected mag. The shimmy from flying on one mag is not that easy to feel in the Raven II. In flight its not always possible to know what MAP should be on a given day, in those specific conditions, so doing the check on the ground is highly recommended.
Mention of low inertia is an interesting one. This comment gets rolled out regularly as a criticism of Robinsons but in this case, its the high inertia of the R44 blades that meant the RRPM didn't come back quickly enough once he'd over pitched. Flying too low to provide sufficient time to recover RRPM is unwise, Get away from the ground asap.