Sarah's laughter

I was wondering why Hashem asks Avraham about Sarah's laughing at the promise of a child instead of addressing Sarah directly. Is a husband accountable for his wife's behavior? Just this morning I came across a fascinating Yerushalmi that answers the question (but I don't pretend to fully understand what it is saying). The Yerushalmi (Sotah 29a in Vilna ed) declares that Sarah was the only female in Tanach addressed directly by G-d. Immediatly the Y-lmi asks about all the contrary examples that may be running through your mind, e.g. Hashem speaks to Chavah, to Rivka, etc. In each case the Y-lmi answers that the communication was through an intermediary, even if unstated. So if Sarah was unique in that Hashem spoke with her directly, why did Hashem not simply ask her why she laughed? Why did he address the question to Avraham? The Yerushalmi answers (see Pnei Moshe) that Hashem acted this way not to avoid addressing Sarah, but davka because Hashem desires to hear "sichasan shel tzidkaniyos", because Hashem wanted to hear Sarah's voice! Had Hashem accosted her directly, Sarah would undoubtedly have not said anything in response -- she would have been filled with remorse and accepted Hashem's declaration of her guilt. But because Hashem communicated his question through Avraham, Sarah denied her laughter.

Of course, this just begs the question -- if Sarah really did laugh, why would she deny it to Avraham? And if she didn't laugh, or felt she didn't laugh, what would she have made of such an accusation coming from G-d?