Would you not agree that tips crossing are a result of the outside ski turning at a higher rate than the inside? It is then the cause that we have to look for to correct. Again, would you not agree?
If the skier is (for whatever reason) outside ski dominant or conversely inside ski lazy, then it would be a very good guess that at that moment the resultant turn shape would not be good. Right? Either way, the inside ski has to be moved out of the way to avoid the tip cross. And you've been around long enough to go thru the Wedge Christie I and II, then up unweighted parallel. Do we do that way any longer? Of course not. The modern turn does not use the outside ski to push into the turn followed by matching the inside. The inside ski is guided, and the interaction of that ski in the snow provides a certain amount of resistance that then transfers to the outside which is enabled to turn as well. .
Regardless of whether the skier is in the back seat, leaning up the hill... whatever, the CoM has to be adjusted to ride the skis without interfering with function. An oversimplification, for sure, but true at every level.
So, what causes most issues with new skiers? Loss of control and fear. Right? Why the loss of control? Assuming they are on the correct terrain for their ability, they get going too fast because they haven't learned to use their turns to control their speed. Teaching how turn shape is speed control provides confidence and confidence helps to keep new skiers from doing funky things. If tips are crossing, how are you going to tell the skier how they can avoid this? Gotta move the inside ski out of the way, right? And how will they best be able to do this? If they have their body in the right position as they guide their skis through the shape. You could just tell them to get the inside ski out of the way, but that would be kind of aimless, no? Turn shape provides speed control and purpose.
As a side note, a proper turn shape prepares the skier for carved turns down the road as it teaches the skier patience and allowing the ski to largely move thru it's own path as it shapes the turn. Rotary then blends into edging into pure carved turns, which lead to dynamics as the skier progresses.
The three things that I listed can be adjusted and adapted for every level of skier. As an example, what's the most important thing for a racer? Wouldn't you agree it is line? Even though a racers' line won't be the same shape as a recreational turn shape and is largely predetermined by the course, it is turn shape nonetheless. And because of the dynamics racers and expert skiers experience, would you not also agree that the inside leg must remain active. And lastly, would you not agree that continuously adjusting balance and CoM is essential for the expert or racer? Why would these things not apply at every level?
My approach begins at the snow level. Turn shape. A shape IN THE SNOW. Secondly, active inside ski- Movement at the foot level anchored at the hip. Third- Transitioning CoM... balance to aid function. All of this built literally from the ground up. How can you find disagreement with any of this?