Canthover
I have no experience with the aerodynamics of the EC120 and it's fenestron, however lots of previous with the Gazelle. The Gazelle has a history of fenestron issues but after some exhaustive testing, fenestron stall as it is known was concluded as a non event and more likely just a combination of human error at the controls. In my view this was smoke and mirrors to protect the manafacturer and a situation where the fenestron can stall under certain conditions does exist. In fact on one of my many Instructor upgrades I attempted to prove this with vector description. I couldn't be proved wrong but didn't feel the need to make a huge ass of myself taking on Aerospatiale!
The interesting part of your post for me is that the crew were executing tail rotor control failure techniques, these if got wrong can lead to unusual airflow and dynamics around the shroud which could lead to a stall situation, it's very easy to say that as I wasn't there and can't say exactly what the handling Pilot was doing at the critical moment. None the less, I can add that tail rotor malfunction practice was written out of the Gazelle training guide as it was considered to risky to practice.