I will also point out that Genna Lannister should still be at Casterly Rock, and she comes before Stafford in terms of inheriting it. She also likely has the force of personality to force her claim over his, and the Lannister household would probably recognize it.
Promising Stafford the Rock is not something we actually have any ability to do, because we don't control it. And Stafford could equally just decide to declare for Renly if he's willing to take over anyways.
Promising Stafford the Rock is not something we actually have any ability to do, because we don't control it. And Stafford could equally just decide to declare for Renly if he's willing to take over anyways.