2."A longer tail arm to reduce trim drag"
How?
The longer the tail arm, the lower downforce the tail has to produce to negate the moment effect of wings lift around CG. Lower lift/downforce on tail = lower drag.
Why don't the Austrian put the fuel tanks on the wings? Now put it on the fuselage instead?
Given that the fuel capacity is not so large and this is primarily trainer where baggage capacity is not needed that much, fuselage tank is a good choice. Besides, wings aren't thick as in other spamcans (C172 etc.), so the tanks would have to be pretty wide. And of course, single tank fuel system is more simple - no need for a fuel selector, all you need is a fuel shutoff valve.
3. "Lets the airplane have better longitudinal pitch characteristics in high speed flight as the tail is above the wing wake. "
How and Why?
DA20's tail with its relatively low height is in the prop wash anyway, but I think the remarks were meant for a jet aircraft with T-tail.