My theory is actually quite simple. The baby was disposed of in the river. LE said river was very fast there and the river really is not that far. It would not be impossible for DB to simply walk and get rid of the body, having lived there I'm sure she knew all the shortcuts, dark paths, etc. In a panic a lot can be accomplished. The reason there is no evidence is because what evidence do you leave when you throw a baby in the river? That is why DB was so afraid to go out back IMHO, she was afraid she'd dropped something out there.
Not saying she didn't have an accomplice either, her brother would be my first guess and I'm sure he could hoof it over there pretty fast, again going through the alley's and such.
I just don't see DB being in good enough physical shape to walk from her house and back with a dead baby to the Missouri River. Maybe not impossible but darn close. She grew up in Independence. It's much more likely she's familiar with the Little Blue in eastern Jackson County than the Missouri.
I know I'm repeating myself but the Missouri River is very wide in KC (not Mississippi in Louisiana wide but wide) and the terrain is rugged and not easily accessible (other than Labenite Park, which isn't that far and the most likely entry point I think but would need a car). Not a case of dark paths or shortcuts. As In Da Middle said....it's easy if you don't know the Missouri River in Kansas City to say oh this could happen...if we were talking an LSU student who has gone missing....you could get to the Mississippi River easily in Baton Rouge or New Orleans but physically it's much more difficult in Kansas City. The only place in KC that I know of where it could be easily accessible would be Berkley Park or by Harrah's and again that's impractical for former and heavily traveled for latter.
And if she's going to the Missouri River...her backyard is in the opposite direction based on what I know.
ETA: What alleys? There are no alleys along the Missouri River....