I have nothing against Frisell or his methods because this does make alot of sense and has been proven to work. You are a fine example on how much improvement that can be made by doing this, but the "head voice musculature" is the same musculature that we use for the other areas of our voice too.
The CT muscle tilts the larynx which causes the vocal folds to stretch and thin. Regardless of whether we are in "head" or "chest" voice the CT controls the changing of pitch. This way of training will work to an extent, but you will never become strong or coordinated enough to sing those same pitches in a chesty way if you are always lifting half of the weight.
I didn't want to give a detailed comment because you have been making great progress, but at some point you are going to have to start to add more weight.