It sounds like you feel your treatment is a bit out of your control. But is also sounds like you feel that fear is holding you back from taking control. Does that sound right?
You mention wanting to talk about your assault with your therapist but either fearing how you will feel talking about it or putting it off because of the feelings you have about it. So your desire to face your assault is there, it's just being blocked by your fear. This is a very good sign and means that you are recovering. It may not feel good, but that is ok.
Your therapist may be taking the lead in therapy because she feels that either this is the path you would most benefit from or she sees you struggle with therapy and wants to help you find what is most important to talk about. But if you have your own ideas about what you want to say, then working towards bringing this issue up should be your next step.
Talk to her about your difficulty with speaking up. I know it's been a topic before, but it may need more work. Confronting how you feel when you assert yourself helps you learn how to get your needs met, a very important step in recovery.
It is hard to tell with OCD, PTSD and depression which came first. You can go by which you developed first, but if they were all initiated by the assault, then it may be too hard to tell. I agree with you that mediation may not be necessary if you feel you are able to cope with your daily life without them. The best way to know is if you slowly taper off your medication and see how you feel.
Chemical imbalances are mostly thought to affect people with illnesses such as Bipolar and Schizophrenia. People with PTSD usually are not thought to have imbalances. Depression can be influenced by a chemical imbalance, but studies are still not clear exactly what causes depression. If your depression started at the time of the assault, than most likely it is environmental in cause and not an imbalance.
There is nothing wrong with bringing up your assault with your therapist on Friday. There is no set rule about when to bring up certain issues since it is the therapists job to be sure you are able to cope with your feelings before you end the session. You may still want to try it and see how it goes.