As far as I know, it's the Hawks who want to move Horford, not Al who wants to leave. I imagine Atlanta could sell Horford on the fact that they were a playoff team without him and have both cap space and young prospect to improve. I don't see them moving him until 2015 at least, which is why I consider him a Spurs' target. He's probably the most unrealistic of the potential Tim replacements, though.
As I've said before, I don't think the Spurs will try to keep the same "system" once Duncan retires. I think they'll go with a more-dynamic offensive option next to Splitter and avoid a plodding twin towers if they can. The only reason why they run the offense they do now is because Duncan can't defend out in space well enough to consistently play a stretch-four next to him. Playing Splitter and Duncan makes for a good defensive combo, but the only reason it's truly viable is because Duncan can flash GOAT talent in spurts to score. Even from a spacing perspective, Duncan's midrange is not reliable enough to keep Splitter on the floor.
It's also worth noting that Splitter and Marc are going to be slowing down over the next few years. That combo will quickly go from being elite defensively to being porous as their agility goes.