Is he who cast the pebbles on the twelfth day and departed Mina, intending to hasten, then returned to it at night, considered a hastener, or is he required to spend the night and cast the pebbles?

You would be in a state of haste, and you are not required to spend the night or stone the pillars after hastening. Sheikh Ibn Uthaymeen was asked about a pilgrim who departs Mina before sunset on the twelfth day with the intention of hastening, and who has work he will return to in Mina after sunset. He replied that such a person is considered to have hastened because he has completed the Hajj, and his intention to return to Mina for his work there does not prevent the hastening, because he only intended to return for the work assigned to him, not for the ritual.