Nick, in your diagram, you provide the TR Thrust acting out of the tail, but do not set the MR thrust acting from the hub.
I hate to provide another voice of dissent, but I'm also not swayed by this argument. I think I can't get my head around the prospect that the MR thrust acts from the hub, at an arm from the CG, just like the TR Thrust acts from the TR hub at an arm from the CG.
I'm happy that these forces act -about- the CG, but don't see why they act through it. And since you won't get a turning moment if they act through it, only a vector, well, this is where I don't get it I guess.
Thus I lean to Arm's point of view that the two couples - MR Vertical Thrust component and weight acting at CG versus TR thrust and MR thrust Horizontal component - provide the roll, and then balance each other out to provide the angle at which the helicopter naturally falls.
Hence with varying weight, you find it rolls varying amounts.