To answer all these questions and more, I strongly reccommend JF's book "A view from the hover". Simple laymans explainations of lots of phenomena.
"intake momentum drag" sounds hideous but is relatively simple to understand. A jet engines thrust is actually a NET value, a summation of all the forces in the engine. The front fan creates significant drag, as the airflow entering the fan has to slow down considerably. This creates a force ahead of the centre of gravity, which, if the aircraft is allowed to yaw, will create an unstable situation whereby the force creates more yaw, which generates more force.
Of course, this yaw at slow speeds is combatted in the harrier using jets of bleed air, if the yaw force exceeds the reaction jet force, you are outside of restoring control to the Aircraft.
Thats how I understand it anyways!