The trup on va'ani

On the Mahpach Yahoo leining group, Rabbi Rich Wolpoe called attention to a dispute as the the correct trup on Bereishit 48:22. The trup most of us have (including Shadal) {Update: As Mississipi Fred MacDowell pointed out to me, this Chumash was published after the death of Moreinu veRabbenu HaRav Shmuel David Luzzato, and so does not necessarily reflect his views on the particular trup} is as follows:

Note the gershayim on the word va`ani. But apparently R' Wolf Heidenheim has a revii on that word. Is there a distinction is parsing between the two? Absolutely. The Gershayim would split the phrase ending with tevir, while a revii is on equal footing as the tevit, and would split the phrase ending with tipcha. That is, up to the etnachta is:

 וַאֲנִי נָתַתִּי לְךָ שְׁכֶם אַחַד עַל-אַחֶיךָ

We would split at the tipcha first, it would seem. This would yield:
 וַאֲנִי נָתַתִּי לְךָ שְׁכֶם אַחַד || עַל-אַחֶיךָ

Now examining that split-off first phrase, we have a tevir alone which can split. And so we would split at that point, yielding
 וַאֲנִי נָתַתִּי לְךָ || שְׁכֶם אַחַד

Finally, the gershayim would work to split the first portion of it, giving
 וַאֲנִי || נָתַתִּי לְךָ

In contrast, with a revii in place, the word va`ani gets split off much earlier. Ideal would have been to have had a zakef on the first word, such that it would be absolutely the first thing split off. But here we have:

וַאֲנִי נָתַתִּי לְךָ שְׁכֶם אַחַד עַל-אַחֶיךָ

Once again, the tipcha kicks in first. Therefore, it yields

וַאֲנִי נָתַתִּי לְךָ שְׁכֶם אַחַד || עַל-אַחֶיךָ

Then, we have a tevir and a revii, both on equal footing. In order to have both split a clause ending in tipcha, the trup mark which occurs earlier in the pasuk must take effect first. Therefore, we split at:

וַאֲנִי || נָתַתִּי לְךָ שְׁכֶם אַחַד

Finally, on what is left, the tevir kicks in, and so the final split is:

 נָתַתִּי לְךָ || שְׁכֶם אַחַד

This is thus a very different parse. And while I can see the benefit of Wolf Heidenheim's revii parse, I don't know the provenance of this trup pattern. Also, if syntactically, va`ani is not first split off, but rather the various prepositional phrases from the end of the clause, then it makes sense that this pattern would continue until the end. But we should examine just how often the gershayim parse actually occurs.

The picture of the parse with gershayim -- that is, as we have it -- looks like this:

And the picture of the parse would there have been a revia would have been: