2. Phantom pertains directly to Zelda the character, not the series. For all intents and purposes, Phantom is a generic enemy included in only two titles. Assist Trophies are usually important characters, or characters that offer something unique in terms of game-play. Phantom is neither.
1. Every Assist Trophy reveal has been a separate daily picture, it wouldn't be rational to reveal a character or Assist Trophy within a character reveal. It was Zelda's reveal, not to be attenuated by the inclusion of a mediocre Assist Trophy.