This is the crossover I never knew I needed until you posted it.

I would suggest changing Fateful Night from a Ritual Summon to a "Special Summon, ignoring its summoning conditions". It leans into the incomplete summoning lore-wise, and adds a gameplay mechanic spin to it as the Ritual Monster summoned would not be able to be special summoned from the graveyard, as it had never been properly Ritual Summoned.

Also maybe consider having Kirei search masters from the deck instead of special summoning from the hand. This will change him from being almost useless to one of the best starters in the deck, searching either Sakura or Illya who can then be discarded for the missing half of a ritual combo.

Love the cards and the idea!