OK, sorry, but that's still got nothing to do with longitudinal dihedral
You're using the trim balance to deduce the c.g. position relative to the aerodynamic centre and neutral point (for the special case when the zero lift pitching moment is zero). But there's nothing, again, in any of that that relates to the setting angle of wing and tailplane.
I happily agree that if the cg is aft of the wing AC, as the tail lift for trim increases then the aircraft is also less stable. But it's less stable because the cg is further aft, not because the tail lift has increased. The tail lift is changing because of the cg movement, but it's the cg movement that causes the change in stability, not the tail lift change.
The original case put for LD is that it's the lift itself that is the lift ratio (or angle ratio) that causes stability. What you are now talking about is tail lift varying as a symptom of the variation in cg that also causes the stability to change. That's not the same thing at all.
The CLt=0 case is very important. The proportionality argument goes haywire at that point, yet in reality doubling the tail area of an aircraft with zero tail trim lift will make it very much more stable - something the proportionality argument cannot address.