Some people swear that point guards are born, and not made. I don't know if that's true, but I'm pretty sure you don't learn how to run the point if you're being played as a shooting guard. I hope that whether it's in garbage time, or in Austin, the Spurs stick Murry at the point and just wear his ass out, to see where he can go with it. Given what we know about Pop, Murray isn't likely to see much daylight in the playoffs. So his job this season is to learn - and he might as well be learning PG skills.
I totally agree with you that the best thing he could do for himself is to focus on learning to play defense the right way. I don't know how much he knows about the Spurs, but hopefully some of the older players will convince him of that. If he really, really blossomed on the defensive end, it would be the one situation where I wouldn't mind seeing him be moved to the 2. A good perimeter defender who can drive and score? That's a bankable package. The Spurs could go look for a backup PG elsewhere.
I saw enough in that last SL game to be hopeful for him. He's not an NBA-ready, lottery pick type, but he'd stand a chance of playing on some lesser teams. Pretty sure the Spurs won't be looking for anything but development from him this season.