You do not name Taichi - do you think it can't be him at all?
Well, I may be wrong of course, but I don't feel it can be Taichi, because on Venite Adoremus the high notes are both powerful and long.
Taichi does high notes on many songs, like on Sanctus, of course Salva Me, and also on Dies Irae, and many more. But appart for Salva Me, he is quite always accompanied by someone, Leo, or Leo and Adam. When he started as high notes performer a while ago and was alone, at times it was not easy to always hear him distinctly.
Adam - where have we heard him? I can't remember at the moment
He contributes on high notes on various songs, mixed with others. it occurred that he also performed a bit alone some months ago, I don't remember well when it was... maybe Korea Spring 2016, or Japan May 2017... And I remember that during our usual concert debrief, we fans agreed about the great quality of his high notes, auguring good possibilities to have him more exposed alone in the future (which don't not really occurred so far).
The Hope CD was released in May 2017, meaning the recordings were probably done somewhere in 2nd half 2016 or start of 2017 for the latest (not speaking about Benedictus Deus of course). A that time, who was able to perform such strong and nice high notes ? Perhaps Alex M, Gregor can be added to the list of possibilities ?
So far my guess is inclined to Rocco, but likely we will never know the official answer
