Dago:

What Ryan is talking about is exactly what I said to start with, in different words.

If replacing the vortex generators with plain tip plates doesn't remove the tendency to flat spin, increase the size of the dorsal gusset until you have no further improvement, then if there's still a problem do the same to the stab.

Yes, if you would rather, you could increase the rudder area. The gusset is easier. More rudder area might work better, though.

Flat spin recovery requires the tail plane to be outside the air disturbed by the wing. That's why you can power out with a prop plane, the propwash blows the tail "Free" of the vortices, but low to medium power jets have to be knocked out with flaps, dropping the gear, or anything that can radically alter the airflow. A high power jet sometimes can be blasted out with thrust.

Twin in a flat spin? Wait, see how far it'll dig in.

Bill.