They released Blackgate, not Arkham.  Those crazies aren't really crazy.

Joker would have driven into that explosion on a jet ski and come out with Batman riding on the back seat. He'd be singing "Only You" to Batman and offering him some dolphin shish kabobs.

Is that the case?  They put the mob and their henchmen in Blackgate.  They didn't say anything about anyone in Arkham.  And they never mentioned that Joker was the lone person in an asylum the size of a small island (Arkham was most of the narrows).  And as far as Scarecrow goes, he was crazy, but he became a joke character.  Reason he was in the beginning of Dark Knight was to play him off as a joke, he knew what he was doing since he was drug dealing, nor did they mention or show the schizophrenia he developed.  Most of the "crazies" plot that was setup in Batman Begins was essentially abandoned before Dark Knight began filming (There hasn't been any other mention of Arkham in Dark Knight or Dark Knight Rises).

I'd Imagen Joker actually saving gotham from the bomb! He doesn't want the city to be destroyed - not like that. He'd offer Batman help, just because it would pain Batman so much to except it. But reading it now, to myself it really sounds silly :p

With how the Dark Knight Rises was written there really is no place for the Joker because I don't think the Joker would be happy without being the center of attention/chaos ...

Hmm that actually sort of makes sense, he wouldn't want the complete destruction of gotham like LoS do, so he would just prefer the chaos and inhumanity that has ensued once people were left lawless. Stop the bomb, and continue to corrupt gotham and make it his "playground" in a way that Bane did, without actually using a bomb to wipe it off the face of the earth.