Soseki has been dragging its feet so much and for so long on this manga, the group got sniped. I'm still waiting for its release, but I'd prefer Soseki focused on this manga as opposed to pulling an LH Scans or worse. Since it's been so long, I think I'll post my speculation about Allen's soul core here.
Simply put, I think that Allen's soul core is what Allen himself became over the course of those hundred million years...but rejected in order to remain the ordinary Allen you see. At one point, after just 7000 years (in chapter 1.2), he forgets who he is and why he was training in the first place (and forgot about his mother). What he did end up remembering was beating Doriel (the future degenerate). The rejection must be subconscious: in order to deal with Icicle Boy (I forgot his name), he exerted himself so much that the soul core fully took over; only when the soul core nearly killed Ria did Allen stop him from beneath the soul core's consciousness. When Allen fought Ria's androgynous guard (don't remember her name either, she has no worth), he exerted himself massively once again--and this time, started to enjoy the strife and combat; this caused Allen's soul core to surface a bit--which caused the guard to defecate in her pants.
All that power is there, in him. It's just that if he embraced it, it'd be exactly what his soul core wants--for Allen, who created him, to give him back the body in which he was given existence.
Then again, there is the legend of the mural to consider, but it doesn't show anything about a time-dilating button or an old man who'd give it to someone and give rise to that demon of Allen's, either.