Mart
Intake momentum drag is rather simpler than you perhaps think. It is a simple drag force that acts at the intake and is caused by the high velocity air that is travelling from nose to tail having to suddenly loose some of this longitudinal momentum as it starts to move radially in the engine. The snag from a Harrier pilot’s point of view is that this drag force acts parallel to the relative airflow and so not necessarily along the aircraft long axis. With any side slip there arises a component of the IMD acting across the long axis and since the intakes are ahead of the CG this tries to yaw you tail to wind.
Yes, the anhedral is there to reduce the excessive rolling moment due to sideslip that would otherwise exist thanks to contributions from sweepback, the fin and rudder and the high wing combined with a mid CG.
John