One thing to understand is that this trade hasn't been announced. It's been leaked. That means that it's very possible that it's being misrepresented. Obviously, they don't even know who's coming back in the deal from Atlanta, which to me implies that the "source" who leaked the deal has been silenced. If the deal were finalized (fully agreed to) and not contingent on anything, I don't think either side would really care if it got leaked. But leaking tentative info leads to situations like this, and I think that's why we haven't heard more.
As far as breaking the agreement goes, I don't think it's a big deal. Besides it being very possible for the two teams to have explicitly agreed to the contingency (so no "Sorry Bud, change of plans" talk would be need), I doubt Atlanta were planning on signing those guys anyway. They've sat on cap space before, and I think they're probably looking for another bargain value as it is. It's also possible they feel Splitter is so good that he's worth waiting on (the same way the Spurs and other teams feel LMA is worth waiting on).
Another thing to consider is that the possible compensation for Splitter may depend on LMA's status. The Spurs might be willing to let him go regardless, but Pop and Bud agreed that if the Spurs don't end up needing the space, the Hawks will pay up more for him. That can also explain why we haven't heard anything about the assets coming back to the Spurs yet.
Seems like there's too much mystery to say that this trade is finalized yet. I think it's very possible that the agreement sort of fizzles out if LMA picks someone else. The Spurs will move on to West, and Atlanta will grab another guy from Europe to be their fourth big.