Oh wow. I just read through 30-40 pages of shitstorm since last night, didn't I?
Anyway, I guess I could weigh in on a few things with my opinions/headcenon.
While
Remember the New Guy was present, it didn't really bother me at all. Nobody really cared when Sweetie Belle was revealed to be Rarity's sister a good while after the introduction of both characters, so I don't see why it should be a problem now. I'd also say that it's been at least implied that Twilight's family is nobility, considering the fact that in real life, you don't even get
looked at for prestigious private schools if your family doesn't have either the fame or the money up front to make it worth their while, so having a young princess for a foalsitter isn't too farfetched to me either. Regardless, even if you didn't care for it, it still didn't create any actual plot holes, and actually managed to fill one in (at least in my mind), that hole being why in the world would Twilight even know the Want It Need It spell. In hindsight it appears to be rather obviously an imperfect attempt of replicating Cadence's signature magic.
In terms of character focus, I can't weigh in because those types of things don't bother me at all. So long as no one is acting OOC and the episode is entertaining, everything's fine and dandy. Would it be nice if Twilight shared the main character slot with the other 5? Sure. Did it actually hurt the episode at all that she didn't? No.
Now, as for Cadence, my headcanon is that she is the young Goddess of Love (which confers on her the Princess title). Presumably she would be on equal standing with the other princesses in a few thousand years or so.
In any case nothing ever creates any actual plot holes, no one was OOC, and we've resorted to nitpicking to find any actual problems with it. I'd say that the finale was definitely a success.