I doubt Riley makes a trade for DeRozan using Winslow. They have an irrational amount of love for that dude.
Honestly, any DeRozan trade has to be about goals, as the OP touches on. The Spurs can "get something for" DeRozan any time up until the deadline. They don't need to look into that now. However, if they want to use cap space, obviously they need something lined up. If they merely want to shuffle the decks a little bit, then trading DeRozan, Metu and Mills to Charlotte for Batum, Marvin Williams and a lightly protected first. I don't love that deal, but Mills being in the deal drops Batum's effective salary to $13.9 Million, and I can live with that. Then you follow that up by trading the least favorable of the Spurs and Hornets 2020 picks to Indy along with Bertans and Beli for Bogdanovic and get:
Murray, Forbes,
White, Walker,
Bogdanovic, Batum, Johnson
Williams, Gay, Samanic
Aldridge, Poeltl, Eubanks
It'd be weird going from having way too much guard depth to needing to sign two or three of them in one trade sequence. But they'd conserve their MLE to sign a decent third-string guard to compete for minutes and could actually bring Weatherspoon over rather than forcing a stash. If they do the MLE, the best use of it may be to take a flier on a highly touted guard who's fallen out of favor on a one-and-one deal. Dunno if there's a good example of one of those this summer, though. Maybe Burks?