Great post. I would like to offer a slightly different perspective on the subject.

We, as incarnated beings, view reality from a very physical, linear perspective, based on our five senses. From this perspective, it appears that we may incarnate and die, reincarnate and die, reincarnate again, die again and so on until karma is cleared. Our Soul/Higher Self, exists on a higher level that is beyond space/time or non-linear. The Soul contains all the knowledge and wisdom attained from every incarnation into the physical reality (and beyond physical). From the perspective of our Soul, these incarnations are all happening simultaneously, with each individual incarnation receiving the requisite information for the fulfillment of that incarnation's objectives/lessons/mission. Souls of a certain group may choose to incarnate together in a multitude of mind/body combinations, to assist each other and fulfill whatever objectives a Soul group may hold.

A particular incarnation having a mature spirit ("old soul"), would be a person having an incarnation that possesses the wisdom attained in other incarnations. Tapping into past-life memories would really be, connecting with the Soul/Higher Self and accessing the experiences of other incarnations that happening right now, elsewhere in the timestream.