"I’m really worried about the subset of the population with preexisting mental disorders such as manic depressive disorder, schizophrenia, and depression who will suffer a psychotic break from the use of these substances."

Psychedelics have been used to treat schizophrenia and depression before. Here is an excerpt from a paper on "Treatment of Childhood Schizophrenia Utilizing LSD and Psilocybin"[1]:

The first patient we treated [with LSD] was an eleven-year-old girl who
was dying of something akin to marasmus. Marasmus is a condition of
wasting away of the body, where nutrients are not assimilated. It
occurs in infants who have no human contact. They go into such an
isolated mental state that not even intravenous feeding is effective..

The girl was actively suicidal and would make every attempt to destroy
herself. She was in twenty-four-hour restraint. When let loose, she
would batter herself: smash her eyes, smash her head against the rails.
She looked like an old, wizened lady -- nothing but skin and bones, and
battered black and blue.

So the psychiatrist said, 'Why don't you start with this girl. She's
going to die anyway, so there is nothing to lose.' It was quite a
challenge to take as my first patient someone who was in such a terrible
situation. I was afraid that maybe she would die during the session.
But I didn't have any alternative, because this was who I was given.

We didn't use low doses with any of the patients. My guess is that she
got 300 or 400 micrograms [of LSD], and she weighed maybe fifty-five
pounds. She was just skin and bones, and was totally incoherent. She
screamed, babbled, and spat if you got near her. She was not in contact
with anybody, nor had she ever been since she was hospitalized.

During the session she started wailing like a wounded animal -- it was
the most chilling sound. Then she started screaming, and the pitch
would increase and increase. We tried everything to make contact with
her, to no avail.

After about seven or eight hours I was exhausted, and so frustrated that
I just yelled at her, "Nancy! When are you going to stop screaming! I
can't stand it anymore!" [laughs] She stopped and looked at me. This
was the first time she had made eye contact with anybody, and said, "I
have a long way to go, so just stay out of my way." That was the first
thing she ever said to anybody. Then she went back to screaming. That
was our session.

Hers was an amazing story, which we subsequently published. Once she
started speaking, it was possible to work with her more effectively.
She was a very intelligent girl, perhaps an ancient soul. She was a
challenge -- conniving and manipulative -- and she gave us a run for our
money. Her bottom line was always that she had control over hurting
herself; this became her ace in the hole. As she improved, she would
eventually just carry a Kleenex around, put it on her hand, and say that
the Kleenex was stopping her from hitting herself. So we would grab the
Kleenex and hide it, and she'd say: "Damn you! Damn you! Give me that
napkin!" There was a whole power struggle that went on for months.

Of course, during this time she was no longer in restraints, and she
could move about on the ward. She struck up a companionship with one of
the other girls in the program, and she was in constant contact with the
staff.

She must have had eighteen or nineteen sessions spaced apart by two or
three weeks, and every session was productive. Between these sessions,
we had to develop a whole treatment program for managing her, as she was
extremely bright and incredibly manipulative. One time she said to me,
"Well, I can't fool the day staff, but I can still fool the night
staff." Then as soon as she said it, I could tell that she was
thinking, "Oh my God! I've given myself away!" I looked at her and
said, "That's not going to last very long, is it?" And she said, "Damn
you!" So, that night I got together with the evening staff and clued
them in on what to do, because she was still manipulating the hell out
of them. Talk about a tough patient! But she was no longer hurting
herself. And although she remained in the hospital, she got well enough
to attend school.

[1] - http://www.maps.org/news-letters/v07n3/07318fis.html