Yeah, after looking at some of the games programming in an effort to understand how some scenes worked so I could mods them, the way story advances are made are kind of a mess. If Marty wanted to do anything with books 1 and 2 I'd bet he'd just rewrite them from scratch; that's a huge long shot though.
As for going back in time and visiting the girls, I think that would be a bit too convoluted for the epilogue since we're so light on story and so heavy on the sex scenes. Sure, the "fixing Korra's legs" thing has really dragged on, but there's a lot of scenes already and only the flimsyest of story to string it along and make some scenes work. Creating some time travel scenes just for that would be a lot of extra work.