If a muta’ajjil (pilgrim who hastened on or left early) returns to Mina, is he still regarded as a muta’ajjil?

You are still regarded as having left early, and you do not
have to stay overnight or stone the jamaraat again after having left early.

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have
mercy on him) was asked: 

If the pilgrim leaves Mina before sunset on the twelfth day
with the intention of hastening on and he has some work in Mina to which he
will come back after sunset, is he regarded as having hastened on? 

He replied: 

Yes, he is regarded as having hastened on because he has
finished Hajj, and his intention to return to Mina to do some work does not
mean that he cannot hasten on, because his intention was to come back for
that work, not to come back for Hajj rituals.

Al-Hajj wa’l-‘Umrah,
question no. 10.