Cecilia Bartoli sounds absolutely amazing. But yes, I do believe that she has to do what she needs to do in order to produce a particular sound that's pleasing to most if not all. Kathleen Battle [a] is on the brighter side and very forward resting on her hard palate. Though her facial expressions could probably directed traffic alone, it seems to not interfere with the production of the [a] sound. Justin Timberlake's and Morris' [a] vowel is more [uh]. I strongly believe that a more conscious [a] vowel could keep the integrity and beauty of the word "hallelujah". However, I'd probably still listen to it over and over again.
Sidenote: Here's a video of Kurt Moll playing Osmin in Die Entführung aus dem Serail.