Parshat Zachor: Derabanan or Deoraita

So, in the second half of Rav M's lecture, I noticed that he understands the machloket whether Parshat Zachor is derabanan or deoraita differently than I did. I spoke to him briefly afterwards and indeed he takes a more subtle approach and I'm going to try and do it justice.

Both the Sefer Hachinuch and the Magen Avram(או"ח תרפה:ב) describe the debate about Parshat Zachor, whether it is Miderabanan or Mideoraita. I took this at face value to be saying that either the sages instituted it as a way to fulfill the mitzva of remembering Amalek or else it is simply the Torah method to fulfill the mitzva. There are several problems with this understanding:

• The Mitzva, as stated in the Torah, doesn't sound like it involves reading this exact parsha
• There was clearly some involvement of the Sages, at the very least in setting the time for reading Parshat Zachor, since yearly torah reading is a much later minhag
• The Gemara in Megilla 18A that I quoted in the previous post learns halachot of Megilla reading from Parshat Zachor. If Parshat Zachor is mideoraita, then how can we learn from it to Megilla reading, something that was instituted later

Rav Markus gave a more subtle explanation of the debate, both sides of which involve the participation of both the sages and Torah law. 

• Chazal instituted Parshat Zachor along with the other 4 special parshiot. One does fulfill the Torah commandment to remember Amalek but ultimately there are other paths to fulfillment and any verbal retelling of the story should suffice.
• The Torah commanded us to remember Amalek, but left it to Chazal to decide how. Chazal instituted Parshat zachor as the means and so it is now the only method to fulfill the mitzva. This concept of תורה מסרה לחכמים is also used to explain issur melacha on chol hamoed and שאר עינוים on Yom Kippur.