I use hypnosis all the time be it just to sleep at night or with in my therapy sessions.

You will not be able to do anything you wouldn't normally do when fully aware

And

?You will be able to say anything that you normally would say when aware.

You will be able so say no but you will not be able to lie because lying is a conscious thought and hypnosis is basically funtioning at the unconscious level of thoughts. But there will be no need to worry about lying anyway.

If you are asked a question that you don't want to answer you will automatically become fully aware again basically instantaniously you will snap awake out of the hypnotic trance.

Besides a person who is lying about something does not submit themselves to being totally relaxed because they will be too worried about disclosing that lie. It is basically impossible to place a person who is lying about something under hypnosis to the level of unconscious level of awareness. For a person to reach the level of relaxation needed for memory work they need to be COMPLETELY comfortable with the process, the the therapy professional and so on.

If you think there is something you do not want disclosed and will need to lie about then now is not the right time for you to be using hypnosis during therapy.

As for healthy to relive those things - that depends on each individual person, sometimes remembering helps sometimes it doesn't.