1. The given sentence cannot take a conjunction because there is nothing to conjoin. You could *replace* "after" with a conjunction, but that completely changes the meaning. 2. The question has no answer because conjunctions don't depend on, or have anything to do with, which preposition is being used.

In your example, I would not use a conjunction. After the guests had left, I went to bed. another way to write: I went to bed after the guests had left. in my opinion, both are correct, and both do not need an additional conjunction.