I fail to see how it is substantiated by the regulation.
That's because it's not substantiated by the regulations! It's rubbish to say that a CRI without IRI can't teach while operating under the IFR. It would be quite correct to say that he can't teach applied instrument flying skills.
It's been a great debate but I feel we've come to a point where we are all entrenched in our opinions and so, as they say in Dragon's Den, I'm out!
Happy landings
3 Point