Well, let's see, progress of sorts. I guess. In an effort to get more than than that silly 1mm of right thrust, I drilled and filed out the holes a bit more. And as an strength improvement added a 6mm think "flange" of fiberglass reinforced material (similar to but tougher than G10) below the ply mounting shelf. So now the ply "shelves" are sandwiched between the metal engine flanges and the fiberglass material.

And so yes, I can now get more right thrust (probably up to what we see in Merlin's photo)...but...it's not exactly equal rotation. Measured on my cable the left side moves forward about 1/8 inch while the right side moves back almost 1/4 inch (both relative to a neutral mark). Obviously the engine isn't rotating around a center point.

And I'm going to make myself not care!