With that, I can't figure out how it's to Redcloak's advantage to have stolen the phylactery, rather than destroying it (and still giving Xykon the fake, of course).
Do we know that liches can survive the destruction of their phylacteries? As far as I know, RAW is ambiguous on whether or not liches can exist without a phylactery. From the d20srd's page on liches, it would appear that the only necessary condition for the permanent destruction of a lich is the destruction of its phylactery, and one reading of "as a rule, the only way to get rid of a lich for sure is to destroy its phylactery" would make that into a sufficient condition as well.

For that matter, I'm pretty sure that the comic is also ambiguous as to what happens if Xykon's phylactery is destroyed; one way of interpreting O'Chul's actions in #655-656 is that O'Chul believes that destroying Xykon's phylactery will destroy Xykon, and I don't think that there's anything in the comic that contradicts that interpretation. Of course, I don't think there's anything else in the comic that directly supports that position, either.

Chucking the thing in a rift would probably be the best idea.
No one knows what'll happen to Xykon or to his phylactery if it goes into a rift. If the phylactery survives going through a rift, Xykon might be able to reform within the rift-world and Redcloak probably wouldn't know about it. "Redcloak moved or switched my phylactery" is also the most obvious answer to "why am I not reforming in my astral fortress where my phylactery is supposed to be." That being the case, and assuming that Redcloak is reasonably smart, getting rid of Xykon's phylactery in a way which only might prevent Xykon from reforming and coming after Redcloak is a bad idea, especially if that way prevents Redcloak from determining the status of the phylactery or from becoming aware that Xykon is reforming prior to Xykon returning to the world.