I don't agree with the premise of the original post. It is not in Odo's nature to allow a genocide to occur when he has the means to stop it. He would be unable to do something so fundamentally contrary to gis nature as to say screw you to the Founder, or to obey a corrupt Federation order not to deliver the cure. Odo's nature demands that he acts in a way that delivers justice to the best of his ability, and he cannot violate that. Therefore, what is proposed in the original post could never reasonably happen.
Aside from that, his "return" was handled in a murky, hackneyed way, much like his out-of-character response to the Sisko's corrupt order not to deliver the cure (ie: him agreeing to do nothing, and then doing nothing) was. Odo could just dip one foot into the Great Link, cure a few, and then leave them to cure the rest of each other. Odo could be said to have "returned" after having done that, or stayed for a few days or a week after having done that, then went back to Kira. Therefore, WYLB making a big huge deal out of Odo "returning" without even explaining the terms of that "return", which doesn't really make any sense in my view. Certainly it's not implied in the show that Odo would be a prisoner forever at the Great Link, therefore, with what is given on-screen, there is nothing at all to stop Odo from having his cake and eating it too. Curing the Great Link for a few days, then going back to DS9 and keeping his hot babe.
In my view all of the above speaks to the rushed and tacked on ending of the series, which despite much praise for the final chapter, has many flaws such as these due to being rushed and tacked on.
