Droopystop: do the maths. Start with the formula in equilibrium (on both sides obviously) and then enter a reduced AofA into the formula on both sides. Everything else in the formula is same.
The CHANGE in "L" on the advancing side is LESS than that on the retreating side, swing round 90 degrees and you get nose down.
Also:
If one utilises the CofG thrust diagram, which you talk about..
If we looked at the horizontal thrust vector emanating from the rotors - then lowering the collective would reduce this vector which is now LESS than it was before, which is proportionately less than the (parasite) drag horizontal vector (in the couple), consequently the nose will tilt UP.
Try again.....................