Does the Valsalva maneuver require a completely closed airway? How do the effects compare to exhalation against partial resistance?
The eustachian tubes exit into the nasal cavity superior to the glottis.  Thus restricting the glottis while exhaling cannot increase pressure in the nasal cavity / eustachians.  Only by closing the nostrils can that be achieved, and only while exhaling.  Even a technique like circular breathing cannot maintain pressure in the nasal cavity since that's necessarily the entry point for every new breath, be it mouth or nose.