preparing for pesach- Chodesh Nissan

(Beis Yaakov Haggada)
There are two opinions in the gemara when one must start asking and expounding on hilchos pesach.
Mishna- Tanna Kama 30 days.
Rebbe Shimon ben Gamliel- Rosh Chodesh Nissan.
The Tana Kama makes sense, as we see that Moshe Rabenu taught the laws of Pesach Sheni on Pesach. Thirty days before. The Zohar says that thirty days before a Yom Tov, there is an announcement in heaven and the gates open up.
So what is the reasoning for Rebbe Shimon ben gamilel? According to him, Moshe Rabenu stood on Rosh Chodesh Nisan and was telling the Jews about Pesach. Why? He is of the opinion that the last 15 days of Adar still relate to purim, as Hashem is giving Bracha to us, and we don't recognize it at all. Once Rosh Chodesh Nisan occurs, the moon reveals itself, and Hashem starts to show the person our rescue from Mitzraim, therefore a person begins to ask and expound (lidrosh, can mean expound or pursue) 14 days before Pesach. The moon means the ability to recieve and reflect, only when a person is receiving can they begin the process of asking and seeking out the laws of Pesach.
The last four days, when the Jews were commanded in Mitzraim to get the lamb, they needed to prepare through action, not merely speech.