Yeah, I think the idea is not necessarily that the Ring of Void becomes the Ring of Life at some future point in its timeline, but rather that one of its enchanted properties is that every time it disappears it reappears as the Ring of Life, and vice versa. Though I think whether that makes them the same ring that changes states or two rings that cannot coexist is more of a quibble. I imagine the similarity to the cherubs is quite deliberate; their relationship to each other is really just a matter of perspective.

As others have pointed out, the last guy to be seen with it was actually Caliborn, who banished his juju chest into the Furthest Ring with it. Aranea's legend only says that the Ring of Void came from a void session, so I've wondered if that was misdirection: did it originate in Caliborn's session (since a dead session is a kind of void session), and pass from English to the Condesce to Droog? Caliborn says he doesn't know where his future self got it, though, so maybe not. It could well be that Calliope (or Meenah) lose their life ring somewhere on Earth and Caliborn picks it up as a void ring in the far future (realising the cherubic nature of the two rings).

In any case, the fact that it has an obscure origin and end point is all part of the point - that's what makes it so magical and therefore alike to a juju, as danya says.