The mana value of a double-faced card in your graveyard is always the mana value of the front face.

X is 0 when determining the mana value of a card in your graveyard.

"Five or more mana values among cards in your graveyard" means there are at least five different mana values among those cards.

The mana value of a split card in the graveyard is the total mana value of both halves of that card. It does not have two mana values.

The mana value of a land card is 0.

Costs to activate an ability are determined only once, before mana abilities are activated. If cards are removed from your graveyard after that point, such as by exiling Jack-o'-Lantern from your graveyard to pay for its mana ability, this won't affect the cost of activating Sewer Crocodile, even if that reduces the number of mana values in your graveyard to below five.