The WK2 really is a fabulous aircraft as Burt have said and it is really very sad to watch again the pilot struggling to keep the draft parallel to the runway on the take-off as it is quite noticeable on the last released video.
It is obvious that the longitudinal instability is primarily caused by the inward engines attached too close to the fuselages. When any aircraft with engines in full power at relatively low (take –off) speeds gets to a side slip even the high velocity engine gases are slightly declined by the free stream airflow. When WK2 gets in such a side slip the outside surfaces of the windward fuselage and vertical tail get exposed to the engine gases. This exposure increases airflow velocity on these surfaces, increases under-pressure and thus generates horizontal lift in the unwanted transversal direction incrementing the yaw and destabilizes the aircraft.
The vortex generators reattaching a separated airflow do not help much as these are usually used to fix the quite opposite problem. Theoretically the best would be to move the engines further from the fuselages or attach the engines on top of the wing middle section but this is practically impossible as the aircraft frame structure is finished (I assume the best position on top in the middle of the wing was very possibly rejected due to maintenance accessibility to the engines in aircraft’s every day operation life). The remaining option seems to be to modify the tail itself. There shall be something invented to decline/detach the airflow from the outside tail surfaces to spoil the unwanted lift when aircraft gets in side slips. Maybe some kind of permanent leading edge slats separating the airflow in negative angles of attack on the outer vertical surfaces would do.
It will not be easy to fix this problem and I’m crossing my fingers hard for Scaled to manage it very well.