I'm almost certain that his contract is up after this season, so at that point, it's 100-percent his decision to be in SA or not. I doubt after three years of waiting relatively patiently, he'll decide to forego the millions again. And I don't think the Spurs want him to stay over there anyway. He'd probably be a much better player now had he come over in 2013 (not counting the ACL of course). He has the talent to be Ibaka-lite, and he's just rotting away over there.
But you obviously bring up a good point about the roster. The Spurs just have so much depth at the middle positions that I just can't see them all being on the roster. Simmons, Anderson, Bertans, LJC, Lalanne to go along with Green, Kawhi, Manu, LMA and Diaw is just too many.