You're confusing two effects.
Dagors are known to shift focus on stopping down. This should cause the image on the GG to lose sharpness.
Most, not all, lenses' gain coverage and, for a while, sharpness on stopping down. This because stopping down reduces some aberrations, off-axis ones in particular, until diffraction, which increases on stopping down, overwhelms the gains from reducing off-axis aberrations. And yes, stopping down increases depth of field.
As an aside, I've always wondered whether all Dagors and dagor-type lenses shift focus on stopping down. This because (a) Goerz Dagors' were manufactured for many years and were recomputed from time to time and (b) my Boyer Beryls, which are Dagor clones, don't shift focus on stopping down.
If you have a 4x5 camera, use it to take two test shots with your 12" Dagor, one wide open and the other at your intended shooting aperture. Set the camera up at an angle to something like yardstick (if you're going to be shooting close) or a brick wall or picket fence (if you're going to be shooting at some distance), mark the subject so you'll know where you focused, and go.
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