I found a way to tell which way Nana will throw when on platforms (may be known).

[Shoobop]

Side Platforms:​

So i have been practicing my hand-offs and i am doing so on Battlefield. I have noticed that if you are one of the two lower platforms Nana will actually throw towards center stage. Although if Nana were to throw them and she was on the edge of the platform closer to the edge of the stage she will still throw them off of the stage as was previously assumed.

This Same concept works on Dreamland. It also works on Yoshi's story, but to get nana to throw towards center stage you will have to be past the halfway point of the platform favoring center stage. And Fountain of dreams' platforms followed the same parameters as did Yoshi's story but with more leeway in favor of throwing toward center stage, where as the direction depended on the halfway of the platform. Aswell same concept from Yoshi's story's platforms apply to those of Pokemon stadium, although you have to be a little more forgiving since these platforms are significantly smaller.

Top Platform:
​

The top platform was easier to work with as it was a constant, with of the halfway point of the platforms differing the direction of the throw. So for example if Nana were to throw a falcon standing on the right side of the top platform Nana would throw them to the right side of the stage, and this same concept works for the left side of the platform.

 

[Cervidae]

From what I've read, I think when Nana is near two edges she favors back throw over forward throw. What you found seems to agree with that statement. It's like you repeated an experiment, which isn't a bad thing; coming to your own conclusions is excellent!

 

[Grim Tuesday]

Nana always throws towards the closest ledge when she's in range of a ledge. Because her back stretches further out than her front - for lack of a better way of describing it, she'll usually bthrow on platforms because the "closest ledge" is behind her

 

[DerfMidWest]

tl;dr:
closest ledge.
when in doubt, she backthrows.