In English, she opened and she would have opened are two very different things. .. Am i to believe that in Italian there is only one tense to express this?? She opened is saying that i believe that this act took place. She would have opened is used when i believe the act did NOT take place, but for logical reasons should have.
You're right, "I thought that she opened the door" is really saying "I thought that she had opened the door" and requires the past perfect instead of the imperfect subjunctive - I was thinking of a situation more accurately described by saying "I was thinking that she was opening the door" (the action occurring at the same time as the thought). Thanks for helping me think about it.