The Magic Mirror

• WWizards of the Coast2019-10-04
  Official Ruling
  Adventurer cards aren't instant or sorcery cards while they're in your graveyard.
• WWizards of the Coast2019-10-04
  Official Ruling
  If a split card is both an instant card and a sorcery card, it's only counted once for The Magic Mirror's cost reduction ability.
• WWizards of the Coast2019-10-04
  Official Ruling
  Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to change by how much a relic's cost is reduced.
• WWizards of the Coast2019-10-04
  Official Ruling
  The cost reduction ability reduces only the generic mana in the relic's cost. The colored mana must still be paid.
• WWizards of the Coast2019-10-04
  Official Ruling
  If The Magic Mirror leaves the battlefield while its last ability is on the stack, the ability will use the number of knowledge counters The Magic Mirror had before leaving the battlefield to determine how many cards you'll draw.
• WWizards of the Coast2019-10-04
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Rulings via Scryfall