Hi, Dan,
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.