It can't be the target of Sorceress's Schemes, which reads in part "Return target instant or sorcery card from your graveyard or exiled card with flashback you own to your hand." When playing a card as an Adventure, use the alternative characteristics and ignore all of the card's normal characteristics
[2] After drawing cards, you can alter your opening hand (also known as a mulligan)
That is, until you take into account the cards first ability - The Power of Evil, which allows the player to play Chernabog, Evildoer for one less ink per character card they have in their discard
Because replacement effects are considered before Purphoros is on the battlefield, the mana symbols in its mana cost won't be counted when determining this
I can't make Vivi or Sephiroth cheaper, but I can try to make sure the rest of the cards in your Vivi or Sephiroth deck don't break the bank
Now lets get to the cards themselves