Right, so the problem is that you can’t tie across staves without moving the note to one or the other staff unless I’m missing something. In other words, the tie can only go across, it can’t twist upwards. (I’m trying to only change things that ultimately make the psalm more readable, not create a totally different thing…)

@MatthewRoth Enter the tied note in the bottom staff, select and press Ctrl+Shift+↑. Then tweak the control points of the tie to get the curves right.

I am zero organist, but an experienced pianist. For me, that notation is weird: in all the quoted cases it would make more sense to leave the tied note in the bass staff. But it isn't. What does it convey to an organist?

Serviam, the flex and any changes (i.e. extra syllables) to the mediant are at the end of the music, and changes to the final also follow. If you do Sunday Vespers weekly, you’ll figure it out, but it gets more challenging when the rubrics call for something else or if special occasions come up. Not all psalms have a flex, and if the organist accounts for it in the French parochial style, then knowing that it’s coming in this psalm is better than having to frantically look down the line. (Obviously playing from square notes is ideal. However…)

Giulio Bas’s music is also weird in its way — he does not always provide the Magnificat music after the antiphon, and the key sometimes changes if you do go back to the collected simple tones. He never gives the solemn tone unlike Potiron.

Gerard, huzzah, I think that should do it. Also, it is obvious that I never had to learn treble clef below the staff — I learned bass clef for trombone, then square notes with solfège, then have sung tenor mostly with treble clef (but with solfège instead of notes or only notes on the staff — a few pieces were in bass clef).