Who delivered the final blow?

[Farael]

More likely is the Ring being cut off of a finger, with that obviously only practicable when Sauron was "dead."

I beg to differ as he could have been incapacitated as well. You don't need to be dead to be helpless! I do agree that the depiction of the scene shown in LoTR The Movie is a little off... because we know Sauron lost ONE finger and the only way to hack the ring finger off someone is going through either their little finger or their index and middle finger! Unless, of course, the person carrying the ring tries to point his ring finger at the other (which, if you try, you'll find is VERY awkward)

Yet that does not mean that Sauron was beyond recovery. His physical body was inhabited by a VERY STRONG soul and therefore one could think that he could recover from many things a human could not and yet the soul chose to leave the body after the ring was cut off. That is, at least, what I understand from the books. So as I said before, Isidur and Elendil might have incapacitated Sauron, but loosing the ring is what made his soul leave the body.