I think the siren idea could work, but (like so many things) it would depend upon execution. Just look at the different film versions of Hamlet or Sherlock Holmes.
The idea came to me that a scarecrow might actually be the weapon of a serial killer--some farmer or such in Collinsport (a la Ed Guin) whose mother used to work at Collinwood (before Elizabeth fired everyone). Now he uses this living scarecrow to kill people, enacting whatever fantasies of revenge and/or sex that feeds his warped imagination. Another twist might be if this guy actually has a split pesonality, and his "other" self doesn't even realize he is a killer!
Okay, maybe that's a bit cheesy, but maybe it'd work. Me, I' prefer that he knows his is a monster and loathes that fact, is depressed and nearly suicidal.
Regarding the living Mummy--suppose this was someone (lets give him a name: Ptah-hotep) with an unsual motivation? He's not looking for a lost love or revenge against a Collins nor trying to escape an enemy. Rather, he is looking for a way to die? He already made himself immortal and wants to end it, but can only die under some very specific circumstances--for example, if he can destroy his coptic jars, and he has reason to believe the last one is hidden somewhere at Collinwood?