Parker and Gasol one for one more season
For one year, paying Parker, Gasol and Mills shouldn't be that big of a deal. Mills is arguably the second best player on the team, right now, even if he doesn't start he's been getting an increasingly larger share of minutes at PG season to season. Next season he could very well play more minutes per game than Parker while coming off the bench.
I don't think that it's crazy to think that Parker may move to the bench next season anyway, he'll be 35 and entering his 17th season (19th when adding playoff games). It would create another issue of bench shooting or lack thereof though; Parker, Simmons, Murray, Anderson, all aren't particularly great shooters from outside or are low volume shooters who don't scare defenses. Bertans is the only bench player (projected to be on next years team) that can actually shoot.
I'd doubt Pau opts out unless he wants to play another year afterward. the Spurs could offer him a two year deal then, while saving some cash this season but I think he'll play out the last year of his deal.