Unhappy homes gloom expansion rules
What's a sensible interpretation of the new rules?

I am going to assume you mean the rule you quoted above. Mystery Cards are a new card type that is available in Gloom: Unhappy Homes. Mystery Cards unlike Modifiers, Events, or Untimely Deaths may be placed on Locations. Gloom's card components are mostly transparent, but the parts that are not, cover up previous cards that were played before the current one (and remove those previous effects if they are covered). From the rules on Atlas Games


  You can't play Modifiers, Events, or Untimely Deaths on a Location. Nothing can take your house rule away from you, so enjoy it while you can.


What's the purpose of the pathos points on the blue-themed mystery cards if the residences are unkillable?

Those points count towards a families final total. From the Official Site.


  A Mystery is the only card that can be placed on a Residence (and only a Residence), and can be placed on any Residence as either of your two plays. It gives that Residence's player a special effect and Pathos points that count toward his final Family Value. A Mystery remains even if the requirements for playing it are lost. You may discard a Mystery from your hand as a free play.