Had a random idea the other day that it isn't Joker at all - it's Alfred, and that's how he knows the secret identities and is serving them dinner, cuz that's what Alfred would do as part of his duties.
I had this idea a few issues back. Joker was doing some of his early crimes with twists when the story started. He kidnapped Alfred because it's a twist on when he targeted Bruce Wayne in The Man Who Laughs. I'm guessing that somehow, Alfred was fighting back and knocked Joker's face off, which was coated in a toxin. Alfred, maddened by Joker serum, then assumed Joker's identity, strapping the face, to his own, dressing in the coveralls, and going after the Family himself. Batman has been too distracted and worried about Alfred's safety to notice that it's not the real Joker it's been Al kidnapping them, because he actually does know their identities.
I'm not sure what's under the cloches. Probably a note with "ha ha!"
That would be totally crazy and totally awesome. It makes perfect sense!