Double-Slately is close enough to the real thing for the world to accept that his side still has a Ruler, and good enough to duplicate the accessories Ace made for him. I would be a bit disappointed if he can, so far, do *everything* Slately could do except designate an Heir, so I want there to be some other reason this revelation is going to suck for Jetstone.. perhaps creating an Heir is not an instant title/ability change, and some wrinkle in the order-of-operations for unit cleanup means the Double spell is going to end before Tramennis actually becomes Heir, even if Charlie does honor the bounty on that Archon.
Charlie may be calling with a different offer, however - maybe he has a way to keep the Double active past the usual limits of such a spell.