The variant trup on ad-avod

Shadal writes of a variant trup in parshat Ekev, and sides with our girsa. The pasuk (Devarim 7:20) reads:

כ וְגַם, אֶת-הַצִּרְעָה, יְשַׁלַּח ה' אֱלֹהֶיךָ, בָּם: עַד-אֲבֹד, הַנִּשְׁאָרִים וְהַנִּסְתָּרִים--מִפָּנֶיךָ.

20 Moreover the LORD thy God will send the hornet among them, until they that are left, and they that hide themselves, perish from before thee.

I used the version at Google Books because the one in plain text does not have the gershayim over אבד. The pasuk with trup, and Shadal's commentary:

Thus, we have a revii on עַד-אֲבֹד while this old manuscript Tanach had a gershayim. What is the difference between the two? Well, a revii splits a clause ending in tipcha while a gershayim splits a clause ending in tevir.

Since two trup symbols of equal splitting power split the verse first earlier and then later, and both revii and tevir split a clause ending in tipcha, the second half of the pasuk is to be split as follows:

עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים מִפָּנֶיךָ

becomes, because of the tipcha:

עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים

מִפָּנֶיךָ

Then, עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים becomes, because of the revii:

עַד-אֲבֹד

הַנִּשְׁאָרִים וְהַנִּסְתָּרִים

and finally, הַנִּשְׁאָרִים וְהַנִּסְתָּרִים becomes, because of the tevir:

הַנִּשְׁאָרִים

וְהַנִּסְתָּרִים

(This last instance might not be such a strong division. After all, there are not three words in the clause, necessitating a division. But Wickes writes that when revii is two words away, the intermediate word is always marked by a tevir.)

In contrast, if we had a geresh (or the equivalent, a gershayim) on עַד-אֲבֹד, then it subdivides the clause ending in tevir rather than the one ending in tipcha. The division would then be:

עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים מִפָּנֶיךָ

becomes, because of the tipcha:

עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים

מִפָּנֶיךָ

Then, עַד-אֲבֹד הַנִּשְׁאָרִים וְהַנִּסְתָּרִים becomes, because of the tevir:

עַד-אֲבֹד הַנִּשְׁאָרִים

וְהַנִּסְתָּרִים

and finally, עַד-אֲבֹד הַנִּשְׁאָרִים becomes, because of the gershayim:

עַד-אֲבֹד

הַנִּשְׁאָרִים

The word מִפָּנֶיךָ is first separated off, and that is what happens from before you. I suppose because the עַד-אֲבֹד applies equally to הַנִּשְׁאָרִים and וְהַנִּסְתָּרִים, we should separate off both in their own clause.

Indeed, from what I recall from Wickes' syntactic dichotomy, when a verb leads the clause, you mark off from the end of the clause each segment. Thus, first we mark off the PP (prepositional phrase), followed by the NP (noun phrase). This then makes sense, and our trup appears correct.