This is the one thing I would not do and especially with a young horse.
They are unbalanced enough and the fact that they broke stride in the canter tells me they haven't the balance or power to carry you.
Who said anything about breaking stride? I said if they picked up the incorrect lead.
To ask for a canter from an unbalanced trot teaches them nothing but to rush into things anyway they can.
You seem to be reading things into my post that are not there. I said that when they start trotting faster(a sign that they are heavy on the forehand and unable to engage the haunch enough to push into the canter, they must be brought back before attempting again. I also said that the reattempt at the lead must happen as quickly as you can. I don't remember saying to throw the horse into it from an unbalanced "frame", but simply to rebalance quickly.
Nor would I shorten the striding quickly for a green horse this is rarely possible.
Definitely not in my experience. To shorten a stride is not hard at all, if you use enough leg to maintain enough energy to shorten without loading the forehand. This is something I teach to my green horses before even trying the canter. My horses can do a regular working trot AND shorten the stride and slightly lengthen the stride without affecting the tempo. Until they can do that, they are not ready to canter at all. It is MUCH easier for a young horse to do transitions from a shorter stride. It is much easier, then, for them to push into the transition instead of pull themselves into it with their forehand.
Also horses don't think like humans and what is done is forgotten in seconds so taking your time and getting the best trot before you ask again shows the patience needed for a young horse.
I have found (actually, I learned this from Capt. Heyer also) that horses are much more prone to repeat an incorrect lead when allowed to go too long before asking again. And, since the trophy awarded in the World Cup in dressage is called the Heyer Cup, I will probably follow what I learned from him.
It is the rider's JOB to position the horse for the best results, not the fastest ones.
I agree that you must instill/maintain balance in the horse. What I am saying is not to dwell on the "correction" SO long that the horse forgets that he has even been corrected. It doesn't take twenty times around a twenty meter circle to rebalance a horse who has just missed the lead. If you cannot rebalance a horse within one circle....well, you have just lost a better opportunity to retry the canter depart, IME.