Shadal: A Revii or Zakef on Noach?

Shadal emends a zakef katon to a revii on the basis of a ktav yad, and on the basis of what he knows to be the logic of trup. The pasuk is pictured to the right, from Mikraot Gedolot, with the zakef upon the word Noach.

Meanwhile, mechon-mamre's trup matches that of Shadal's emendation, or rather, Shadal's ktav yad:

וַיָּ֣בֹא נֹ֗חַ וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ אֶל־הַתֵּבָ֑ה מִפְּנֵ֖י מֵ֥י הַמַּבּֽוּל׃

What is the difference between the two? Well, one difference is that in manuscripts, a revii consists of a single dot (rather than a diamond shape), while zakef katon consists of two dots. So the difference is a single dot.

In terms of actual trup, both sequences are indeed possible, if we are to ignore semantics. The difference is rather in terms of how the pasuk divides. With a zakef in place on the word Noach, the first dichotomy of the half-verse ending in etnachta is on the word Noach.

Thus,
וַיָּ֣בֹא נֹ֔חַ וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ אֶל־הַתֵּבָ֑ה

with a zakef in the word Noach, would divide as follows:
וַיָּ֣בֹא נֹ֔חַ
וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ אֶל־הַתֵּבָ֑ה

The latter clause would then subdivide into:
וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ
אֶל־הַתֵּבָ֑ה
This is somewhat weird, in how it severs the verb from the rest of the actors, and from the destination. We should expect to first separate off the destination they all went to, and then the various actors.
In contrast, with the revii in place, we have:
וַיָּ֣בֹא נֹ֗חַ וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ אֶל־הַתֵּבָ֑ה
subdividing into
וַיָּ֣בֹא נֹ֗חַ וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ
אֶל־הַתֵּבָ֑ה

At this point, for a clause ending in tipcha, there are two trup marks which can subdivide it, and so the earlier one in the pasuk subdivides first, followed by the later one. These two trup marks are revii and tevir. So we have:
וַיָּ֣בֹא נֹ֗חַ
וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו אִתּ֖וֹ
and then the latter clause will subdivide into:
וּ֠בָנָיו וְאִשְׁתּ֧וֹ וּנְשֵֽׁי־בָנָ֛יו
אִתּ֖וֹ
and so on and so forth.

Update: See Lion of Zion where he discusses some of the manuscript evidence for one over the other.