Is Corruption His?

In Haazinu:

ה שִׁחֵת לוֹ לֹא, בָּנָיו מוּמָם: {ס} דּוֹר עִקֵּשׁ, וּפְתַלְתֹּל. {ר}	5 Is corruption His? No; His children's is the blemish; a generation crooked and perverse.
ו הַ לְיְהוָה, תִּגְמְלוּ-זֹאת-- {ס} עַם נָבָל, וְלֹא חָכָם: {ר} הֲלוֹא-הוּא אָבִיךָ קָּנֶךָ, {ס} הוּא עָשְׂךָ וַיְכֹנְנֶךָ. {ר}	6 Do ye thus requite the LORD, O foolish people and unwise? is not He thy father that hath gotten thee? hath He not made thee, and established thee?

Shadal has a compelling theory, that this is a conversation. And that pasuk 5 should be parsed as:

שִׁחֵת לוֹ -- Israelites at time of trouble: Corruption is His!

לֹא בָּנָיו מוּמָם -- Israelites continue: The blemish is not to His children.

דּוֹר עִקֵּשׁ וּפְתַלְתֹּל -- Moshe: A generation crooked and perverse!

This would well account for Moshe's condemnation in the next pasuk, of why they are trying to ascribe this to Hashem. For the Bnei Yisrael have contradiction HaTzur, Tamim Paalo.

Of course, this goes against the trup, and the Targum, as he notes. The trup is pictured to the right. The tipcha is the first division of the clause ending with etnachta, and the tevir subdivides the clause ending with tipcha. Thus, the division is:

שִׁחֵת לוֹ לֹא בָּנָיו מוּמָם

becoming

שִׁחֵת לוֹ לֹא

בָּנָיו מוּמָם

and

שִׁחֵת לוֹ לֹא

dividing into

שִׁחֵת לוֹ

לֹא

Shadal explains this as the trup and Targum trying to avoid making the people listening to the Shirah have to hear this blasphemous remark.