differential of Cl by Alpha !!
you are close but wrong (to agree with Crab on that point)
you say: "So only valid for a given point on the rotor."
whereas I think you'll find it's valid for each point and therefore equally true when integrated for all r
(there are of course (always) second (of third) order effects that are not relevant to the essential thrust of the observation)
(Cf is Centripetal Force not Centrifugal acceleration, and you are being ridiculously nit picking. wrt an aeroplane wing folk talk about stall speeds, or even sometimes stall angles, but of course it isn't true everywhere on the wing, but it is nonetheless a useful concept in an aeroplane, similar to this concept in a helicopter. And yes I do mean 'Accelerated Stall' wrt FW FWIW)