A beautiful piece to listen to, not just for the music itself but the purity of the voices. But the music is so accomplished that comment is barely appropriate. You asked about the counterpoint. I learned species counterpoint and though this in rather a different league from V, I looked at the Sanctus in a modicum of detail. If you have broken any rules I didn't spot them - besides, nothing was apparent in the sound itself. But I can hardly claim to be an expert. I noted the fugue form - that alone takes some mastery - you turn to it often (if not a fugue then imitative entries). And the modulations were handled always with skill (it was something with which I always had problems). I noticed the way you marked phrasing. I'd guess your study went deeper than just looking at the famous renaissance composers' scores - as if you assessed how they were really sung and captured that in score.
Altogether it had a beautiful solemnity about it.
Edit. A propos your declaration "While the final product did not disappoint me, despite having employed all my knowledge and skill, I knew I had not produced an authentic piece of 16thCentury at all. " If true you seem to have come very close. I doubt WIlliam Byrd could have got much closer.