Not sure if it fits, but you may want to consider a 3rd option for the Dark Brotherhood. As it stands right now, you have two paths, both of which require you to do what the Aretino boy wants.
1. Kill one of Astrid's captives, join the guild, etc.
2. Kill Astrid, and follow the alternative to wipe out the Brotherhood.
I'd like to see another path where you meet up with the kid and the "I'm not who you think I am" dialogue option bails you out without assigning the quest and optionally allows you to turn him in to the town guards.
As much of an evil old woman as Grelod the Kind is, you shouldn't have to be forced to go through killing her to be able to wipe out the Brotherhood. Turning the kid into the guards could even instigate the portion of the quest where the courier brings you the "We Know" note, thus presenting an alternative way for Astrid to confront the player about the stolen contract.