My opponent plays a Melancholic Mercurius and plays its multiple 2 ability, on the 2nd multiple's E he plays Mentally Unstable's E, and upon dealing damage, he takes back Melancholic and its 2 multiples into his hand from his card pool. He then plays Melancholic again, does it have the bonus from the earlier Melancholic played? Or since it's no longer in his card pool, there's no bonus?

Effects, to the best of my knowledge unless stated otherwise end when the card leaves the card pool and/or staging area.

The effect from the previous MM will disappear when it leaves the card pool.