To clarify, by agreeing to three-year deals with Collins and McD, the Spurs left themselves the option to acquire those players in a S&T. That means, they could either wrap those players up in a deal for DeRozan or trade someone else for them. I don't really have a lot of time right now, but the Spurs could definitely acquire Collins, Collins and McDermott and S&T DeRozan for something else in the same off-season. That particular combo would require them to trade Murray or White (or some other combo that gets them to the $15-Million range). But if we were talking about like a Simmons trade, that might be necessary anyway.
One example:
Trade for J Collins to give ATL a TE
Trade DeRozan and whatever for Simmons
Trade Murray into the ATL TE while simultaneously trading with Portland and Indiana for McD and Z Collins
Simmons, Jones,
White, Vassell, Primo, Jeffries
McD, Johnson, Wieskamp
Collins, Samanic, KBD
Poeltl, Eubanks, Collins
Would probably let Jeffries go for a vet PG using the room exception.
Again just an example. The Spurs haven't even committed to being a cap-space team yet. They have a lot of options on how to structure their deals.