You have literally no proof aside from being "aesthetic pleasing" which really doesn't matter, who stops to look at the roster? No one, they select a fighter and play, also, we already had separate pages for the stages(new stages and past stages), why the same isn't "aesthetically pleasing" for characters?
The proof I have, however flimsy it is, comes from the following observations.
1) The character order between the two CSS displays is different(Greninja and Pikachu secifically).
2) 48 cells fits very cleanly on the screen, includes Random, and can be adapted to deal with DLC by way of using a separate page.
3) 48 cells
also fits very well on the Wii U CSS as well.
4) Roster math. 33 known confirmed, 3 remaining from Gematsu, 11 MIA veterans(following likely cuts such as Snake, Ivysaur, Squirtle, Lucas).
5) Mario Theory.
6) O12.
I think Zoroark is the one I'm leaning for in this argument. Even though I'm having trouble following. The demo was obviously an EXTREMELY old build of the 3DS version (Wii U as well, but we're talking 3DS). It makes sense to have character selection on the bottom screen. You can drag your marble over your character. I would not be surprised if they did that.
There's also the possibility of the top screen's roster scrolling Horizontally instead of Vertically as you guys keep saying. Considering the 3DS top screen is widescreen. It would be more polished it it did this.
The point is. There's far too many possibilities for people to be sure there are 48 characters. I personally am inclinded to believe there's 51. It's more pleasing on the Wii U screen.
If the bottom screen were taken up by characters, how would you see who has picked what? How would you see who the CPUs were, or see their AI settings/handicap settings?
While there's nothing we can say for certain, you're right, there is evidence beginning to show itself that allows for the formation of a hypothesis in regard to not just the roster size but also its layout. I'm not saying this is gospel, but I see a lot of things pointing to 47 + Random, at least, to me, far more than to support 51 + Random or more.