Can't really see the problem of demonstrating valve drills kneeling. It's a complex skill which requires the students utmost attention. For clarity it makes sense that the instructor is not moving and can show all the critical attributes of the skill. Most of the tech instructors I've met demo this on their knees, even at the trimix level.
I am simply stunned by the last sentence. I have never seen ANYONE do a valve drill on their knees, not even me. If you cannot do a valve drill in such a fashion as to emphasize the critical attributes, while hovering in good trim and position in front of your students, in my opinion you have no business whatsoever billing yourself as a technical instructor. I mean, really -- I can do this!
I think putting a 10 dive diver into a set of doubles, sitting them on the bottom of the pool and letting them twist the knobs is a travesty. I do think an avid diver, who has logged some bottom time and wants to learn to use doubles for redundancy for deeper or longer dives, should be able to take an "Intro to Doubles" class if he wants to -- but even then, by that point, the diver should learn his skills in the water column. Perhaps not in perfect trim (I think it took quite a few of us a while to get our rigs balanced well and to master the body control for perfect trim) but managing solid buoyancy while doing valve shutdowns or air-sharing. If you can't do those things, you aren't ready to add anything more complicated to your diving. You should stay with your current gear, and take classes (or practice) until you can manage neutral buoyancy, good trim and position, with the basic drills of your diving level.
Encouraging people to move on when they're still not competent basic divers is a disservice to everyone involved.