The stuff in here about planning for them to get engaged and for Catwoman to leave him is really stretching disbelief. He had to just know somehow that Batman would propose to her just from them teaming up again, even though they've known each other for years and it hadn't happened. How could he possibly KNOW that Batman would propose? And that Selina would leave him just from what the Joker and Holly said to her? I mean, they could have easily just talked to each other about this and worked it out, and then the plan is shot to hell. Or they could have run off and gotten married before the Joker and Holly managed to say anything to her.
It's not a very foolproof plan.