Lantirn,
In addition to the wealth of information provided above, I'd like to point out a common error in do-it-yourself aerodynamic calculations. You have forgotten to account for various airspeed instrument and pressure error. The calculation for Va (actually, Vo according to information above) is Vs times sq rt of limit load, but where Vs and Va are both expressed in knots EAS. EAS is very similar to CAS for light aircraft.
The stall speed of a Katana at max weight at sea level with cruise flap is 54kts CAS. Do the maths on that, and you can see why the published Va of 104 is actually quite a conservative number.
My own experience with the DV20 leads me to conclude that the published Va is limited by rudder movement rather than elevator movement. It is a very large rudder supported by a long and slender aft fuselage. But I really don't know for sure, any more than anyone else outside of Diamond's design team...