Ha, I started thinking about that as I was writing this, partially because I've been reading about Roman imperial succession lately. My suspicion is that there were several people Caspian was considering naming heir, probably old friends and trusted subordinates (I don't expect Drinian to have outlived Caspian, but I bet he was at the top of the list) -- grown men, anyway. He was holding off on making a final official public decision until right before he died, just in case, but there were probably documents drawn up before he set sail at the beginning of SC, just in case he didn't survive the journey. And then Rilian comes back and probably there is a small civil war, ehehehehe.