On the pragmatic focus and emphatic force of „aber“ in a sentence

Question

I read this example in Duden’s Richtiges und gutes Deutsch:

Sie hörte Schritte, es öffnete aber niemand.

1. Which sentential constituent does aber focus on: the verb öffnen or the subject niemand?

2. According to Duden, the adversative force of aber is least emphatic when it is posited immediately after the finite verb, and the example quoted above is just used to illustrate this point; however, I think the construction es öffnete aber niemand reads more emphatic than the normal construction aber niemand öffnete. Would you agree or disagree with me on this point?

Answer 1

In this case "aber" focuses on the verb "öffnen". As she heard steps, she was expecting somebody to open the door, but that did nlot happen.

Your second question seems very dependent on personal opinion to me. I would neither agree nor disagree, as both cases can sound more or less empathic depending on accentuation.