1. You could luck out and flip over the crown/ring (and win).
Thanks for the reply! Thought this will only happen if my opponent has already 12 cards, no? Because 0 power cards (thus the crown/ring) will go to their hand.
This is correct. If the crown/ring is revealed by Seer, it doesn't automatically end the game. The card would go into your opponent's hand, unless they're over the 12 card hand limit (which would make them discard). The obvious benefit of this is that you then know where the card is, and it should be much easier to challenge to win the game.
As a side note: in all cases, simply seeing the crown/ring isn't enough to win the game. The rules say that the game ends when someone has to discard the crown or ring, and discarding only happens because of a challenge, not when a card is viewed through a card's special ability like Seer or Frigg.