Sorry to hear of your struggles. I too am working to change embouchure settings to a more Windworks “Milkspout” setting. After a lesson with Greg, he noticed my jaw was closing and this was causing me trouble with lower note sound and resonance. I am currently working to engrain a lower more open jaw into my Windworks process practice but am still a “work in progress” and have a distance to go yet.
That being said I am wondering if a focus on a jaw down/open teeth position would open up your oral cavity to allow more room to lower your tongue arch. I do not use KMT tonguing but tried to emulate it and found a lower jaw did free up my tongue to drop more easily from an arched setting.
Just a thought and a suggestion.
Good luck and please keep us posted on your progress.