I'm with your POV. Even in the initial run -- I saw her both in LA and on B'way -- she vacillated greatly in her placement of various pieces of the score, sometimes resorting to thin but pure head tones, other times going for an almost shouted belt. Interestingly enough, for this character, both work. So much has been written about ALW's composition for this character, initially Frank Rich claimed that the composer boxed the role in a corner, the tough singing canceling out a whole list of actors who could play the hell out of it were it not so ... well, ALW. Close, the most famous, has actually disproved Rich's case, to a point, and performed the role as an actor, singing, to be sure, but always making choices that served her needs -- and Norma's emotional expression -- in the moment.