It can still all be explained in future updates. Lucario being back says nothing about Sheik at all. If anything, Pokémon Trainer is in a tighter spot now. And like Sheik, Pokémon Trainer is a character with duo mechanics. Jigglypuff was low priority in Brawl to, so not sure about her this time, at least not 100%. And if she could be cut, Sheik would be easier.
About the Pokémon Trainer: Yeah of course. Let's remove the only real playable character of the Pokémon games.
Sure, changes gend to be mentioned. But we still don't even know how many Pikmin Olimar can have for example. Many things are left to be revealed about the excisting confirmed cast. Not even most veteran's whole special moveset has been revealed either for example. Brawl had Special Moves for character updates, and Sheik was revealed when Zelda's Special Moves update happened.
"Brawl had", as you say. This is not Brawl's Dojo. Pit's changes, the only known real change (not properties only, for example), were shown during the develloppers direct, and in June and July for the daily pics. An additionnal information about the Upperdash Arm invincibility frame was shown in september. Maybe it was considerer during that time?
Phantom could be explained whenever they want. It's likely they'll still do silly updates about Rosalina's other specials, or something random as Link's Side B, DK's Side B, other Special Moves for Zelda and Dedede. Marth's Counter / Dancing Blade...
Rosalina = Newcommer, so it's obvious they will show what she is able to. It reminds me that in one of the offscreen gameplay video, with the Megaman VS Mario battle, there also some Wii Fit Trainer informations, and we can see she lack an animation of turning back, around 2 minutes. She simply slide, while standing still. I know she have a good balance, but still
http://www.youtube.com/watch?v=Iktxi6tPBPw Also, the website, her video, the devellopper's direct AND this offscreen gameplay lacks two things: 2 special attacks. Althought one of her, the down special probably, was shown in the daily pics, but the effect is unknown. And still, it comes from a time where the Sakurai's comments contained less informations.
It's likely that when something is finished, it's shown. Of course, you can't show everything. Between stages, items, Assist trophies, Pokémons, characters, their animations, their attacks, yet doing sometimes some pictures only for entertainement, you can't show EVERYTHING at once.
Now, the phantom clearly exist in the game. Why such a change in Zelda's moveset would be left unexplained, yet show something as trivial as the graphical change about her side-tilt?
Doubt Sakurai would be balantly stating Sheik would be gone if she did though. He never made any mention to the cuts of Melee characters specifically at the Brawl Dojo. Why should he do now? Am thinking he'd want to reveal Impa shortly after, so Impa could very well be the next newcomer to be shown.
I doubt it, considering Sakurai seems to seperate the art style in two: realistic and Toon, with Alfonzo replacing the SP Toon Link when WW Toon Link is on the stage. By this, he seems to keep characters coherents: what belong to the Toon art style belong to the toon Art style, what belong to the Realistic TP style belong to the Realistic TP style.
Of course, this is personnal assumptions, and the SS characters proportions are enough realistic. But the game itself have something a lot of people tend to not realise: it's in pointillism style. I really liked that graphical effect, that gave the game a painting feeling. And hide some graphical flaws, at the same time
In all honesty, I wouldn't had been opposed to Impa if we had the Skyward Sword Zelda (and the Ocarina of Time Ganondorf, to represent more Zelda games), but now that we have two Twilight Princess characters, Sheik, even if only being a concept art in this game, is a better choice.
Anyway, I still think it can go either way. Even if Sheik's in, Impa still has a chance. Rosalina was always considered less important than other Mario characters, and too similar to Peach yet she made it in fully unique. Impa and Sheik could still co-excist like they do. Similary to Kirby and Jigglypuff for example.
Well, the differencies between Peach/Rosalina and Kirby/Jigglypuff are way more important: Of course, Peach and Rosalina share the same art style, but their role are different: Peach is the Princess of the Mushroom Kingdom, and the Damsel in distress, Rosalina is the adoptive mother of the Lumas, and protect the cosmos. Kirby and Jigglypuff don't even come from the same game.
Now, Sheik and Impa. They both are Sheikah. For sure, we had Marth and Ike, two blue-haired lords with swords, and they play differently. But it's somewhat the same debate than the Chrom one: Fire Emblem has better to offer than another blue haired lord. The difference is that Chrom is the hero of his game (along Daraen).
Same goes with Impa, if Sheik is in: the Zelda serie have better to offer than another Sheikah. And as you stated, Rosalina is considered less important, so it open the gates for other less important Zelda characters.