If you cast a spell for another cost "rather than pay" its mana cost, you can't choose to cast it for any other alternative cost. You can, however, pay additional costs. If the spell has any mandatory additional costs, such as that of Abhorrent Oculus, those must be paid to cast the spell.

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

If you cast a spell for which mana may be spent as though it were mana of any color, you may cast it for Leyline of Mutation's second ability's alternative cost and still spend mana as though it were mana of any color.

If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.