I say they should just sign Richards.
The problem with Europe is that if he's not far enough along then teams will just junk him because he's not in their long term interests, only San Antonio's interest. It's great when De Colo is a legit player and gets real time for an ACB team, but just as often there are different scenarios. Mahinmi was supposed to make great progress when he signed with Pau-Orthez and get to play in the Euroleague, but he was just a bench-warmer. Viktor Sanikidze was with an ACB team, but likewise never played. He had to start over at the garbage Georgian League level like Richards, but it still took years for him to remake his career.
On top of that, I don't see why the Spurs have no problem spending real NBA money and roster spots on projects that they have nothing invested in, turning some of them into real NBA players, but might have second thoughts about doing the same for someone like Richards.
They can pay NBA money and put NBA coaching into guys like Gee or Tolliver, but not Richards, who already has had a very bad stretch in Europe? It's just the 15th spot, and there's not luxury tax at play here unless I'm mis-informed about how close they are. There aren't a lot of good candidates for mid-year buyouts who would be able to come in at the all-star break to improve the roster, not the last time I checked anyway.
Just bring him over and let him spend the year in Austin. At the very least you won't have to worry about his nutrition or physical health being what keeps him from developing, as opposed to eating rock soup in Georgia on the Sanikidze diet.