OK, how about this attempt to nail the proportionality argument:
It is not uncommon that you will have a DOWNLOAD on the tail in order to trim. As the cg is moved forward, the download required to trim becomes greater, therefore the tail lift (as a magnitude) also becomes greater.
According to the proportionality argument, increasing the tail lift in this way will reduce the stability of the aircraft, because the effectiveness of my tail is somehow related to the absolute amount of lift it is creating.
While in reality a forward movement of the cg produces an INCREASE in longitudinal stability.
What the proportionality argument is claiming (I think!) is that there is a 'sweet spot' where tail lift to trim is ZERO and any movement away from that point reduces the stabilising effect of the tailplane, whether that movement be forward or aft. This is, I'm sorry, nonsense.