An ability that triggers "whenever you fully unlock a Room" triggers when a door becomes unlocked and the other door of that Room is already unlocked, or when both doors of that Room become unlocked simultaneously.

If a permanent with an eerie ability enters at the same time as one or more enchantments, its ability will trigger for each of those enchantments.

Use Infernal Phantom's power as it last existed on the battlefield to determine how much damage it deals due to its last ability.