Joker definitely switched the addresses. It doesn't make sense for Bats to change his mind like that en-route to saving Rachel.. besides, if he's all "She's waiting for me, Alfred".. why would he let her die like that...Doesn't make sense
Joker wants Dent to live, because he wants to (if Dent lives, and Rachel dies) prove to Gotham that even the greatest of men fighting the crime in Gotham can be turned to evil... That's what the whole end speech was all about between Joker and Bats.. He (Joker) knew that if Dent lived and Rachel was gone, that would be a huge burden on Dent, and of course the Joker would exploit that and try to turn him to evil...which he did...Remember, "...like gravity, sometimes we all need a little push..."
Once Bats got to the location in which he thought was Rachel, yes he realized that it was the wrong place. You expect him to turn around and try to get to the other address? hell no, since he's there and Gordan and the rest of the cops are at the other location saving Dent (but really Rachel), he made the quick decision to get Dent out fast... That's why you hear Dent yelling No! all the time while Bats was trying to save him. They all wanted Rachel to be saved, but they were in a situation in which they couldn't change that.
I don't know if you've seen other movies with interrogation rooms, but they do have microphones and the people outside the room can hear what's going on inside.. That's how Gordan was able to know the other address.. I know that's a common sense thing, but it sounded like some people were unsure as to how Gordan got one of the addys....