I'm not sure why Scarlet Devil Mansion is written Benimakan in the title, unless it's supposed to be a pun on "bean".
It's a silly, intentional misreading, probably just used to make fun of the SDM's name, making the whole situation seem even more silly. It's not a pun though, unless there's something like Benishouga but with beans instead. The 紅 in "Koumakan" can be read as "Beni" but that would never be anyone's first guess when trying to read the name.