Corinne is a regular here, but you could also have added the code {{ScoreError|mm17-18 conundrums}} to the CPDL Mitte manum tuam (Heinrich_Isaac) page while seeking clarification from the DTÖ edition at IMSLP. In the case of m18 the 1550 print has the lower note, the upper being found in this ms.

Chris, can you be specific in what you perceive to be a conundrum, that I may see if it is something I could clarify by updating my edition?

I finally have Finale, again (was without for a good while), and so I can actually start to do some transcription work, since my 5mo is at an age where she can now amuse herself for decent periods of time.

As for the two pitches on the same beat in T1 m18, the source score indicates the upper pitch full size, with the lower pitch in a smaller size placed just to the right of the upper note. I see two possible interpretations of this (since this source is, itself, an edition): (1) the smaller, lower pitched, note may be an alternate possibility to the larger, higher pitched, note (either note would make harmonic/melodic sense); and (2) the smaller note, being placed after the larger note, might indicate a short "passing tone" or even a "liquescent tone" for the "l" in "in-cre-dul-us" (the edition underlay "in-cre-du-lus" notwithstanding. Personally, I tend towards the latter interpretation.

As for the curly symbol under the "in-" in T2 m17, note that in the source edition, the "in" is italicised, whilst the rest of the underlay throughout is not, so this suggests to me that the curly symbol simply points out an omission in the underlay in the original manuscript source.

Not snarky at all, Corinne. I'm glad the thread happened, too. I haven't had a chance to look through my copies of the Choralis Constantinus for possible other instances of such phenomena.