I think there is a difference between stress tolerance and panic tolerance. I don't think there is such a thing as panic tolerance; panic, almost by definition, is the state you reach when fear has overwhelmed your rational responses and you become irrational. What stress tolerance training does is push panic further away, because the individual has learned that stressful circumstances can be managed, and therefore does not end up in the bottom of the incident pit, where they see no answers. However, again, the key is that the individual has to learn coping mechanisms and be facile with them, before they are confronted with the sudden stress that demands that response. That is why technical classes begin with critical skills dives, done in shallow water in benign settings, until the students are exhibiting a correct, competent and confident response to rehearsed and orchestrated scenarios. Only then is the training taken deeper, and the issues made more real.
In the context of open water, the diver is taught a variety of "skills". If the class is a good one, the diver has considered and recognized the type of situation where that particular skill would be called for. After demonstrating the skill for the instructor, one hopes the diver has then rehearsed it a number of times with the CAs (which is what we do). THEN it is fair game for someone to swim up and demand gas, or to demand the diver's mask. Even in the pool, it is unwise to stage scenarios with a high likelihood of a maladaptive response, like for example swimming up to the student and mashing on the power inflator, to make the student remember to disconnect the hose. You'd end up on the surface with a very startled and unhappy student at best, or an embolus at worst!
It takes bandwidth to assess a situation calmly, rifle through one's playbook of responses, select the correct response and implement it. New OW students are critically short on bandwidth, even if they are talented and doing well. All harassment training would do at that point is destroy confidence and create an opening for panic, where none might have existed before.
We had an OW student's yoke regulator come off. She had installed it improperly, and the staff checking gear (no, I wasn't there!) had not picked it up. When she got a mouthful of water, she calmly turned to the DM escorting her and signaled "out of air". Honestly, he was more frightened than she was. No harassment training, but she had a well-ingrained, adaptive response to the particular emergency she encountered, and she implemented it with great poise.