Rainbow Dash's rainboom might have been vital to the well being of Equestria, but if how Rainbow loses determines how Equestria falls apart, than wouldn't the fact that she won with Starlight and Twilight talking together affect how they saved Equestria? It might not be as big of a difference, but I can't help but to think something significant had to have been changed.

Another theory could be that it wasn't Rainbow Dash as much as some sort of Magic energy that might have been created through the multiple trips in time to the same place, Maybe the spell itself comes with magical ripples that become available in different locations?

4886073 Well, at the start of the episode Pinkie was alone, and had cake. At the end of the episode, the rest of her friends were there, but Pinkie had no cake.

Also, the entire cast is now aware of the fourth wall.

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The eighth timeline is basically the same as the original timeline, except that the animators remembered to put wings on some of the foals.

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Another theory could be that it wasn't Rainbow Dash as much as some sort of Magic energy that might have been created through the multiple trips in time to the same place,

I like that theory, myself. It fits in perfectly with what Twilight said: "Friendship connects all of Equestria, and undoing one group of friends made its magic less powerful." The magic of harmony holds all Equestria together; it's what has fought off every enemy from the Windigos on. Once the sonic rainboom shot forth Harmony to tie together the Mane Six, Equestria was safe; it doesn't matter anymore whether Twilight and Starlight were talking together on some cloud.

(You see, friendship is magic, so magic is friendship, and it all comes from Harmony. Or something like that? At least, Sonic Rainboom-level magic.)