Mike, lions will be asking for your bravery putting all this out there. At the huge risk of giving unsolicited advice, and perhaps even more for the benefit of those who haven't taken part in the process, this is what I'd add:
I think you summed it up really nicely in that you only had 20 minutes and introduced three drills that could have interpreted any number of ways and might well have been difficult to coach over an entire day. L3 teach is a distillation, and a pretty radical one at that. Seeing the group, even with different goals that you've asked them about, etc... what is ONE thing you could show them all, that you think would benefit them all, and have fun with while giving direct specific individual feedback to address what you saw in their individual skiing? Maybe the task itself has sorted something out and obviated the need to do anything more than point out the change made and seeing if it can be 'owned' as you move from 'task' to 'till'*. What nugget can they deposit in their account?
Skier 1.... 'sending' a foot out, but bracing.... (which puts her back and in, no?)
Skier 2... 'less flexed' at transition.... (not 'settling', which would allow more ability to allow the femur to rotate through it's full range of motion allowing the ski to move more actively in a 'cross under' edge change.)
Skier 3... 'bringing his upper body down over the hip while still going 'up and over' ..... (This one's not quite clear, but it sounds like there's difficulty flexing the ankle, or perhaps too much pressure directed into the cuff of the boot at the end of the shaping phase of the turn... perhaps some 'following'... just guessing without a visual.)
In 20 minutes, decide immediately and choose ONE of the drills you mentioned. IMHO, almost all the 'standard' drills we know can be taylored with your individual 'twist' that shows understanding, ownership, and ability to communicate to each person in the group the fundimental(s) being addressed with a short, simple progression starting with a body part, a movement, a sensation, a desired change..... static, simple move, complex (more speed, rhythm, turn size, incorporate terrain variation as its encountered, change of DIRT, etc.., and finally into 'normal' skiing. Check for understanding on the fly with each person rather than using vertical hill space and group time. Keep the group moving and alway err on the side of mileage. Each individual can be coached (again, on the fly) according their need and understanding of the information presented. One short, sweet deal, three mini private lessons in that very tight context. Wrap up. Done.
And yes! Terrain management is very very important. Never waste a foot of vertical on the hill.
* 'till'... where the money is!
(the 'rib cage' thing is something I will mention to someone who's having trouble moving toward pressure, but not as end in itself with an entire group, but that's just me... I do think it's interesting and useful, but needs to happen simultaneously with what's happening from the knees down, or feet up.)
Now if this is out of line, please let me know and I'll happily delete this post. I know failing an exam is a kick in the crotch. Been there, done that, was very pissed off with myself and the world. Again, huge kudos to you for sharing your journey. Odd thing is, when you pass and look at it all in the review mirror, you know exactly why you didn't initially succeed. You notice I said "when you pass", because you will!
Mark