If you are not familiar with it, you might want to check out Nanavira's thoughts on the term sankhara in "A note on paticcasamuppada", which is found in
He deals with the Culavedalla Sutta quote which you cite in your original post. He takes sankhara to mean "determinant", or "a thing from which some other thing is inseparable - in other words, a necessary condition". So perceptions and feelings are necessary conditions of any mental phenomenon. No perceptions or feelings = no mentality.
This obviously departs from Bikkhu Bodhi's (and nearly everybody else's!) more context-bound definition of sankhara, but reading through Notes on Dhamma it looks as if he is able to use this definition consistently in different contexts.
Worth a look, anyway, to make up your own mind.