I think you are still missing the two key problems with this proposal.
1) The Spurs wouldn't financially handcuff themselves with Marion's heavy contract, which would make him the second highest paid player on the team. There is no way San Antonio is going to pay him 13 million, only 2 million less than Duncan is getting per season, to be the fourth option on our team. Especially if they have to ship off three or four guys to do it. For a single guy, he's not worth the dough.
2) The guy is a perrenial playoff headcase. He got locked down by Bruce Bowen last year to the point of non-distinction, and pretty much seemed to put himself out of things in 2003. The Spurs, even IF they could afford him, probably wouldn't take it.
This team's iden y revolves around Tim, Manu, and Tony, and I don't think they plan on changing that any time soon.