As I just edited my post to mention, I agree that the adoption would be MUCH better than the retcon. And you are right that it'd be a little silly.
Also, while I love the adoption and every wonderful BatDad and Cass moment we got pre-2006, I can't say I love the "story" because of all the crap that happened. She really should have been adopted at the end of Murderer/Fugitive.