1. The crew is poorly coordinated.
2. Feels like a stall prevention test.
3. Everything is pointing to a rushed job. Time from last normal communication to first signs of a problem is around 10 seconds. You need to set up a stable plane, configured as required. Then reduce airspeed at a maximum of 1 knot per second while maintaining altitude and 1g. slower if there are additional concerns. So assuming 10s at 1 kt/second, they started the test only 10 kts above the assumed stall speed, which is... not professional.
Sounds to me like they rushed a stall test, stall prevention was not properly calibrated, "test" pilot failed to recognize the failure prior to loss of control.
A rush job AND poor communication/coordination without a clear plan is likely what killed these people.
A swept wing, rear engine, cruciform tail airplane has more than a few challenges with very high AOA stall recovery especially with any additional uncontrolled yaw movement.
It's best to have a certified, experienced test pilot conduct these tests, with expert knowledge to the flight envelope limits AND consequences of failure.