If multiple cards would be put into the enchanted player’s graveyard at the same time (due to Millstone, for example), they are instead all revealed and put on the bottom of the enchanted player’s library at the same time. That player chooses what order to put them in. The order is not revealed to the other players.

Wheel of Sun and Moon won’t affect tokens that are put into the graveyard from the battlefield or copies of spells that resolve or are countered. They’re not cards, so they’ll go to the graveyard as normal, then cease to exist.

Wheel of Sun and Moon’s replacement effect will apply to cards that would be put into the enchanted player’s graveyard from any game zone. This includes from the battlefield (if a nontoken permanent is destroyed or would otherwise be put into the graveyard), from the stack (if a spell is countered, or if an instant or sorcery spell resolves), from the player’s hand or library, and so on.