Stall/spin is a leading cause of death among GA pilots and
passengers.

Caused by the pilot not paying attention. Will having another
instrument that he's not paying attention to really help?

Best glide (potential emergency situation) is determined by AOA.

A few knots either way isn't going to make much difference. Plus, how
often is maximum glide range critical in an engine out situation? How
closely is the pilot really maintaining one airspeed (or AOA) during
an emergency?

Put an AOA sensor on GA planes with a hand that smacks the pilot on
the head

Some studies I've seen have shown that pilots are often oblivious to
warning horns and lights, though stick shakers are effective.

My prediction: put an AOA indicator on every airplane in the fleet
and you won't see much change in the accident rate due to stall spin.