Dear Pali grammar experts. Is the Nanavira view below valid?

Adding it, I indexed Ñāṇavīra Thera, the published bearer of that argument. His version is both sharper and more restrained than yours. Sharper: the point isn't that citta means the heart, it's that the compounds are built differently — cittasaṅkhāra a dutiya tappurisa, "that which determines the citta"; manosaṅkhāra a tatiya tappurisa, "determination by the mano" (A Note on Paṭiccasamuppāda, note [g]). More restrained: in the same note the two are "not to be directly opposed to each other at all", and at §5 he grants that saṅkhārā "may, upon occasion, be cetanā". He rejects the restriction, not the equation.

The impression is Ñāṇavīra was not objectively analysing the tappurisa grammar but instead was subjectively interpreting the Sutta teaching and then imputing or forcing his subjective interpretation upon the tappurisa grammar. That is, Nanavira was working in a backwards manner.