Well, I think it is safe to say the Joker wasn't lying about the detonators. Part of his angle is causing terror and chaos.
With the deal with Rachel and Harvey he switched the addresses because he KNEW that Batman would choose Rachel. So what's better than him choosing to save Rachel and ending up saving Harvey instead -losing Rachel in the process? That's the harm of it that Batman would make the "wrong" choice by choosing Love over Logic (saving the much needed star DA.)
With the boat set-up for his "plan of terror" to work he has to make a group of innocent people killers and allow them to live their lives knowing that they made a decision to break society's boundaries and actively choose to kill someone. That plan can't work if the detonators blow up their own boat.
With the deal with Rachel and Harvey he switched the addresses because he KNEW that Batman would choose Rachel. So what's better than him choosing to save Rachel and ending up saving Harvey instead -losing Rachel in the process? That's the harm of it that Batman would make the "wrong" choice by choosing Love over Logic (saving the much needed star DA.)
With the boat set-up for his "plan of terror" to work he has to make a group of innocent people killers and allow them to live their lives knowing that they made a decision to break society's boundaries and actively choose to kill someone. That plan can't work if the detonators blow up their own boat.