The trup on acharei derech mevo hashemesh

Summary: An analysis of the trup on acharei derech mevo hashemesh, according to Rashi, Rashbam, and Shadal. Does Rashi have a complete theory of trup?

Post: In describing the location of har Gerizim and har Eval, the Torah relates:

That is a pashta on acharei, a yetiv on derech, and a munach zakef katon on mevo hashemesh. Rashi writes:

[on the other side of the Jordan, way beyond,] in the direction of the sunset: [i.e.,] beyond the Jordan, toward the west. And the cantillation marks of the verse prove that [אַחִרֵי and דֶּר‏ֶ] refer to two separate things, for they are marked with two [conjunctive] accents [thus demonstrating that these words are not connected]: אַחִרֵי is punctuated with a pashta [which separates the word from the succeeding one], and דֶּר‏ֶ is punctuated with a mashpel [which we call a yetib]. In addition, [the דּ of the word דֶּר‏ֶ has a dagesh inside it [which indicates that the word דֶּר‏ֶ begins a new phrase or topic]. If, however, אַחִרֵי דֶּר‏ֶ were one phrase [meaning“beyond the direction”], then אַחִרֵי would have been punctuated by a conjunctive accent, namely a shofar hafuch [which we call a mahpach] and דֶּר‏ֶ by a pashta [the combination of which indicates the connection between those words. Additionally, the ד of the word דֶּר‏ֶ] would not have a dagesh inside it. [But this is not so, and thus אַחִרֵי and דֶּר‏ֶ are separate words in this verse.]

דרך מבוא השמש: להלן מן הירדן לצד מערב. וטעם המקרא מוכיח שהם שני דברים, שננקדו בשני טעמים, אחרי נקוד בפשטא, ודרך נקוד במשפל והוא דגוש, ואם היה אחרי דרך דבור אחד, היה נקוד אחרי במשרת בשופר הפוך, ודרך בפשטא ורפה:

That is, there is disjunctive accent in the form of both the pashta and the yetiv. Therefore, they are separate, and unconnected. Thus, it means way beyond, in the direction of the sunset.

Rashbam also explains the trup:

פסוק ל 
אחרי דרך - הרי הם כמו שני פשטות, אבל בשביל שטעמו של דרך באות ראשונה נעשה שופר מהפך. 

Thus, he knows that yetiv is on the same level of pause as a pashta, and so explains that really it is like two pashtas. But, because derech is accented on the first syllable, it transforms the pashta into a yetiv. He calls it a shofar mahpach, which would be a conjunctive accent, but I believe he either is calling it that informally, because of identical orthography (though it is placed in a different location in the word). Either that or he believes it is really a mahpach, but that it stems from an original pashta, and so has the same value.

Shadal writes (here, locally, but also in his Vikuach al Chochmat Hakabbalah) about this pasuk in Re'eh, and whether Rashi truly understood this.

He writes that when you have a run of two disjunctive accents, the first one divides more. And so the pashta divides more that the yetiv. (Using Wickes, pashta and yetiv are on the same "level", in that they both subdivide the clause ending in zakef katon. But in order for this to work, the first one, the pashta, must provide the first division.) Thus, it is not acharei all by itself. It is samuch, construct, but not just to the word derech. Rather, it is construct to the entire phrase of derech mevo hashemesh.

(Wickes would agree. Given the initial phrase  אַחֲרֵי דֶּרֶךְ מְבוֹא הַשֶּׁמֶשׁ and this specific trup, it would be divided into:
 אַחֲרֵי || דֶּרֶךְ מְבוֹא הַשֶּׁמֶשׁ

and only then into
דֶּרֶךְ || מְבוֹא הַשֶּׁמֶשׁ

This means that acharei is samuch to the entire rest of the phrase.)

Therefore, since it is samuch, they are connected. Shadal attributes Rashi's differing in this to lack of knowledge in the full field of trup. It seems that Shadal is correct, at least in terms of his parsing.