I simply do not wish to kill Gerolt (this time) since I'm playing a "good guy" and killing that sad old woman is not in his character...so are there any alternatives in order to meet Astrid in that abandoned shack?

not unless there is a mod for it.

Is there ?

http://www.gamesas.com/topic/1407508-is-there-an-alternative-for-the-destroy-the-dark-brotherhood-quest/