Palatal expanders proved effective on adults in a study. is the opposite possible, squeeze dental arch to narrow it? would it narrow the whole face?

Cannot squeeze arch: The upper arch width cannot be decreased with appliances, they can only tip the teeth making the crowns of the teeth slightly closer together. Surgery is the only way to actually narrow the arch.

Specious results: Palatal expanders in in adults do NOT expand the palate, they tip back teeth out to the side, often at the expense of whwt's called the buccal cortical plate, creating fenestrations. You cannot widen or narrow the adult face without Orthognathic surgery. Google the terminology, then consult with a qualified Orthodontic Specialist.

No: generally the palate can not be expanded in adults without surgery, and compression of palates in adults requires surgery to be effective. With expansion efforts or compression in adults without surgery, the effect will only be denta, not skeletal.