At the reading of the accident report, I am surprised to see that the sudden loss of the left horizontal stabilizer resulted in such a violent pitch down.
This incident already has happened few times on AS 365 N family and always a recovery was possible.
I myself experimented it when air testing a 365N2. I was in a +/- 15° dive, looking for a vne when it happened. I noticed slight oscillations in pitch, pretty much as when we encounter turbulences but in an other hand, it took a much longer time to recover from the vne dive. Might the cause of the violent pitch down of this ill fated helicopter, be an aerodynamic stall of the right remaing half stabilizer ?
As for the hydraulic leakage apparently on the left landing gear bay area, I suppose it could have come from a leak of the 2 liters emergency landing gear extension tank. This typically leaves no indication on the caution warning panel and doesn't affect the flight controls.