Need Help With A Teretz

The Gemoro (Megillah 14a) says קרייתא זו הלילא. The reading of the Megillah is considered as the saying of Hallel. Therefore we don’t need to say Hallel on Purim.

The מאירי says, if someone didn't read (hear) the Megillah, he must say Hallel on Purim. (so at least he will be Yotzei Mitzvas Hallel)

The Gemoro (18b) says that R.Meir didn’t have a Megillah on Purim, so he wrote one by heart.

Shu”t Chasam Sofer O.Ch. 192) R. Yehuda Osad zt"l asked,
"According to the מאירי   R. Meir should have said Hallel instead of writing a whole Megillah?"

The Chasam Sofer answered him that saying Hallel is only בדיעבד.

My Eidem R. Yoni asked, that "The Kushya doesn't even begin and the answer of the Chasam Sofer is not to be understood.

All the Gemoro says is, by reading the Megillah you were Yotze the Mitzvah of Hallel. The Gemoro never said  that by saying Hallel you are  Yotze Mikra Megillah."

For R. Meir to be Yotzei Mikra Megillah he needed a Kosher Megilla. Saying Hallel would only solve the Mitzvahof Hallel and not Mikra Megillah.

 הפלא ופלא שהוא