Can the Nine-Tails now leave Naruto's body at will? After Naruto and the Nine-Tails became friends, Naruto unlocked the seal.  Does that mean that the Nine-Tails can escape out of Naruto at his will whenever he wants?  If so, why doesn't he do so?  After helping him, couldn't they both go their separate ways?  If he were to leave, would Naruto die?

And while fighting Tobi in tailed beast mode, Naruto's seal was unlocked. Couldn't Tobi have used his Sharingan to control the Nine-Tails and force him out of Naruto, just like he did with Kushina when the seal was weakened during childbirth?
The short answer:
No, the Nine-Tails could not leave Naruto's body at will.
The longer answer:
Bijuu remain sealed inside of a jinchuuriki until either:

the host loses control of the bijuu inside,
the seal weakens, or
it is forcefully extracted.

In the event that the bijuu is extracted, the original host typically dies.  The Uzumaki clan has had a few temporal survivors of a forceful Bijuu extraction.
The specific example you reference for the Nine-Tails to be taken out of Naruto only really applies to female jinchuuriki; the seal is weakest during childbirth, and given that Tobi had attacked Kushina and Minato while Naruto was being born, extracting the Nine-Tails wasn't a difficult task.
Bear in mind that the seal between Naruto and the Nine-Tails really represented an ideological and metaphysical seal.  The seal prevented the Nine-Tails from fully taking control of Naruto's body and will, and also prevented Naruto from fully influencing the Nine-Tails.  However, after he had befriended Kurama and established mutual respect between them, the seal was no longer necessary.