1. Did I say it was just the harp? No. Try to read more thouroughly. As I've said, none of this is concrete, but it is still very probable that she is a descendant. There is the harp, what the evil ones are after (most likely not the triforce), the fact that this game is supposed to explain some ties between Zelda and the "goddess(es)", and the fact that it just seems to fit well.
2. No, actually they don't mention the triforce specifically nor do they show it. The original, Japanese version carries more significance for obvious reasons anyway even if it did.
3. As I've said, and you should know from the game, Vaati is killed in MC. A resurrection in SS without anything major happening to him in the end would fill a hole in the timeline. In some four swords games released in the future they could have him getting sealed in the four sword, setting up for the original FS.
4. You think that Ghirahim's master will look just like that silhoette in the center? That's depressing. We don't even know for sure that the figure is Ghirahim's master even though it is very likely. And yes, I know Vaati looks nothing like the silhoette. I was just pointing out that they both have horns... kind of.
5. You understand, don't you, that the dark interlopers are simply evil beings that sought after the triforce at one point early on, were held prisoner at Arbiter's Grounds, and were then sealed in some realm that came to be known as the Twilight Realm once the interlopers evolved into the Twili, right? What's the evidence that Ghirahim and the others are the dark interlopers? They're evil, they might go after the triforce, and it would be about that era. Make sure you know what you're talking about before making such big claims. Some people have critisized me for making some big claims without hard proof, but at least I know what I'm talking about and can back myself up.
6. The beasts could be among that which Ghirahim is among, is what I am suggesting. Not that they are the same race. -_-
7. You do understand that every single one of the silhoettes has mismatching ears/horns don't you? There is no reason to believe any particular one is Ghirahim.
8. Just because Vaati used to be a picori, does not mean that he can't be the leader of a group of evil beings. Again, there is no reason to believe that the interlopers have to all be the exact same race.