you are correct in observing that the 85% NROTC Scholarship quota applies to a combination of Tier I and Tier II. Nobody from NSTC has ever commented about how that 85% is broken out between Tier I and Tier II, though I had the impression it was about 60% Tier II and 40% Tier I... only because there are a lot more math/science majors in ROTC than Engineering majors. BTW not all engineering disciplines are Tier I the last time I looked.
Just speaking from my gut and without any real info, I would say that yes, Tier I gets priority over Tier II.