1: While I admit that the laws of comic book melodrama all seem to be pointing towards Eddie having regained his memory and his twisted criminal side, we don't KNOW that this is the case. He might simply have been convinced that his number was up when the bomb-blast hit him, and is going 'YES! I survived!' in his own over-the-top fashion.
2: Assuming for the moment that his supervillainish side HAS returned, may I point out that nobody KNOWS this except him. The world still thinks he's gone straight, or, at least, is no more suspicious of him now than in ever was after his reformation. Now, if he's smart, this gives him a distinct advantage, and a whole new way to play games with Batman and the cops - as long as he's careful, nobody need know that the old Riddler is back. He can taunt the law with a whole new type of riddles - for instance, set up a sort of riddle-crime that ONLY HE can solve, and while the cops and the Caped Crusader are scratching their respective noggins, he can swoop in and 'solve' the case himself (pinning the guilt on someone else, of course), which both satisfies his need to lord it over everybody with his mental acuity and gives him a whole lot of time in the spotlight, with the additional bonus that THIS time, he doesn't have to go back to Arkham! Of course, it's anyone's guess as to just how long he could pull this off, but it could certainly lead to a series of interesting stories. What if, for instance, while he's in the middle of pulling off one of these stunts, someone hires him to solve a casewhich directly conflicts with his own ends, but, for some reason, he also really wants to solve, if for no other reason than his pride? He'd have to juggle his glorious cop-humiliation scheme with an actual attempt to solve an actual mystery - his pride at his abilities as the Riddler would be in direct conflict with his pride at his own intellect. It could be fascinating.