MBC-97 is very explicit in the second sentence of 22.12.4 (p. 177): "If a vowel sound is to be carried from the last note of one music line to the first note of the next corresponding line, a slur is written after the last note and before the first of their respective lines." It goes on to reinforce this by stating, "The tie is treated in the same manner." (We are all comfortable with brailling the tie at the end and a reminder at the beginning of the next line.)
This says to me that a single slur must always appear at the end of the line when the syllable is continued in the next. Regardless of how many notes are in the first line, that line must end with a slur and the new line must begin with a "reminder" slur; this combination clearly indicates that the syllable continues through the line break.
Proofreading some Bach cantata choral scores, repeatedly I have had to go back to De Garmo to read the slur rules. Finally, at about the 30th time, I realized why I was having so much trouble reconciling the difference between 3 notes at the end of the line and 4+ notes at the end of the line as described in the DG text and illustrated in Examples 31-2 and 31-3. I'm forced to conclude that, according to the Code (and logic), the second sentence of the last paragraph on p. 428 of De Garmo, the bold "except the last" at the top of p. 428, and Example 31-2 do not agree with the Code.
I would appreciate it if you would review these texts too, and tell us if you reach the same conclusion. If the DG material is correct, can you please explain the inconsistency in the use of the final slur?
All I can say is that the De Garmo editors thought that an additional single slur is not required at the end of the first line when a double slur is in effect, unless there are not four or more notes of the syllable in the second line. Bettye Krolick wrote the wording of the code, and that's what she said it meant. Let's make sure we clarify that one way or the other in the upcoming revision of MBC.
Karin A. and I have been discussing it, too. She pointed out that the single slur at the end of the line terminates the doubling. After she said that, then the way it's used started making sense to me. Maybe wordings have to be tested on people who don't already know what the words mean?