I'm sorry you are having these thoughts - But I truely believe that the doctors are there to help us.
With regards to the ECt - if you do got that route, can you not get a friend to accompany you and check on you during the treatment? It will put your mind at ease, knowing you did indeed get the treatment.
If you were taking part in a placebo vs. drug test, the doctors would have to notify you (Obviously not if you were taking the drug or the placebo, but that a test was being done).
If you are displaying symptoms that need to be addressed, the doctors are obliged to give you the medication that they feel is most appropriate - and that will NEVER be a placebo.

I'm sorry these thoughts are tormenting you, but kudos to you for speaking to your nurse!