Someone suggested earlier in the thread that the pilot could have been too tense on the controls and prevented the governor from doing its job - that could have caused reducing Nr and subsequent reduction in TR thrust.
Combine that with a little weather-cocking - I'm not convinced by the LTE pie chart - and he mis-diagnosed a TR malfunction, over controlled and crashed. Had he just used full left pedal and waited, he would probably not have ended up in the spin.
Interference between the MR vortex and the TR isn't a new piece of knowledge, we used to teach it specifically in a Max All Up Mass sortie on the Wessex 30 plus years ago - it certainly doesn't cause a spin, just a bit of twitching in yaw which goes when you change the heading in relation to the wind direction - the arc really isn't that big.