another terrible ATPL PoF question

[Arm out the window]

I thought that might be a sticking point - if that's a concern, then replace the ceiling hook with a balloon that just lifts the weight, and a preferably bikini-clad assistant applying the appropriate equal and opposite side force with another spring balance where the bag hangs off it. The equal side forces will stop any lateral motion anyway (as is the case when we apply an appropriate amount of cyclic to counteract tail rotor drift), lift will still equal weight, and the rest of it should work the same.
In that case though, we might as well use the original picture of the helicopter.

As regards whether the helicopter moves around the CofG or not, it doesn't really matter in this case - the basic force diagram should remain the same. Moving the tail rotor further down with respect to the head increases the arm for the lateral couple - same forces and bigger arm equals more torque, therefore more roll.
That in turn will move the head (the suspension point) laterally with respect to the CofG, so creating a wider arm for the vertical couple (lift and weight) and increasing the restoring force provided by it, resulting in a new equilibrium being reached at a greater angle of bank.
I see where you're coming from in saying if the aircraft rotates about the CofG, weight has no arm and therefore won't be a rotating force, but in a complete force diagram, all forces need to be considered, and it's the combination of all that will determine the resulting motion, be it around the CofG or some other point.

Re putting pictures in to posts, I only found out recently, but it's dead set easy - go to photobucket.com, get a log-in name and password, then you can upload pictures to that site.
The site generates a number of references to your picture, and by copying and pasting one of them (the bottom one of three, starts with IMG or something like that, from memory) into your Prune post at the appropriate position, people subsequently reading your post will see your picture, courtesy of the photobucket site.

Cheers