a. It's definitely dark. No doubt.The Apartment is an inspired choice, but I'm not sure truly how funny it is. It gets pretty dark at times. However, it's probably Jack Lemmon's best role and it's so weird seeing Fred MacMurray (Mr. Douglas? Say it ain't so!) being so evil that you have plenty to keep your attention.
One other suggestion is Intolerable Cruelty with George Clooney and Catherine Zeta-Jones. They've never looked better in front of the camera (although you could argue her debut in Mask of Zorro is a photo-finish tie), and the banter will have you rewinding to catch it all, or watching it multiple times. "Have you forgotten Kershner?"
b. Entrapment?