The correct bracha on real Hallel (as opposed to Rosh Chodesh and the end of Pesach) is לגמור את ההלל. The Maharam advised people to instead say לקרוא את ההלל because if you say לגמור and then miss a word, it would be a bracha levatala, and this is what Ashkenazim do.
I was listening today to R' Fendel of the Sderot hesder yeshiva describing how the sirens started in the middle of Hallel on Shemini Atzeres. I seem to recall a similar story in a kinnah describing an interruption by Crusaders (?) in the middle of Hallel on Shavuos, and was wondering if there's any connection to the Maharam's psak. Is there any possibility that the Maharam was making a zeicher to the story and trying to emphasize the kedusha of the martyrs' unfinished Hallel?
(With obvious parallels to soldiers who interrupted their Hallel to fight...)