The Shaagas Aryeh says that nowadays we are exempt from the mitzva of writting a sefer Torah because we are not experts in which words have extra or missing letters and it could make a Torah pasul. Therefore the mitzva nowadays is a D’Rabbanan.Therefor the Chasam Sofer writes that we do not make a bracha on writing a Torah because it might be pasul.
Q1:So why then do we make a bracha on the Torah before and after if it might be pasul?
Q2:In addition, if we are not experts in the area of extra or missing letters then why by teffilin which there is a machlokes of the word totofos(Tur 36)to we have no pproblem,and we wear and make a bracha on it?
A1:the Rama(143:4) brings down that b’deieved you could make a bracha on a pasul Torah.Therefore if we do not know if it is kosher or pasul we can make a bracha in order to read the Torah b’tzibur.
A2:The Minchas Chinuch(613) brings down that klal yisroel are experts in writing teffilin since we do it on a constant basis and we are very careful in preserving the mesora.So there is no question that they are surely kosher.(M’rafsin Igri)