Sheik being in two Smash games with Impa barely getting a mention proves that in Smash, Sheik is INDEED more worthy.
Also, Beast Ganon does not exist in OOT. That's Pig Ganon. Honestly, do your research a bit better. They're different characters completely.
Also, Ruto could not tell the difference. How could she? Or maybe she already knew and didn't say to prevent anyone from being able to recognize Zelda? We don't know why she said that either
Link and Zelda were indeed the same age(well, that we know), which is around... 17 in OOT.
Also, getting Trophy mentions don't mean anything. Venusaur wasn't playable, after all. That was Ivysaur. I mean, you're making my Waluigi theory look legit, and that's just bad. I don't even think he'll be added. I think he has a fair chance, but why would Sakurai do that? When there's better choices.
We know he thinks Sheik is important to the Zelda character herself. Pichu was the only one he did not care about whatsoever. He even thought of making a Toon Sheik. It's pretty clear that replacing her with Impa is way too unrealistic and unlikely. I don't even think she has any real chance of getting her own slot.
There's absolutely no one that'll ever really be cut among the Zelda characters. All 5 of them will continue to show up. I'd like to remind you that if Sakurai could, he'd cut regular Link before Child Link. But that would make no sense, since Link is part of the original 12, is a 3 time veteran, and extremely popular.