I assumed that Vo was an actual speed determined by calculation or
flight test.

Ah. I'm not sure how they determine Vo. They don't specify how it's
to be calculated, and the Part 23 Flight Test guide doesn't say how to
find it experimentally (unlike things like Vmc).

a quadratic relationship between lift and AOA (which is only true
for small AOA & probably off by 10% or more close to the stall).

I assume you meant "between airspeed and AOA" ?