The decision on Thomas belongs to the team. The way I look at it, the Spurs will be paying some new big somewhere in the neighborhood of the MLE next year. Personally, I hope that big is Splitter. Thomas could be a fallback. The Spurs don't have to tender the QO in order to negotiate with him, the QO only makes him a restricted FA. They could also tender the QO and then withdraw it up to a date about two weeks after the moratorium ends in July.
With regard to the wings, only RJ and Hairston are under contract. Even if Manu is resigned, I believe that the Spurs will try to keep Mason rather than filling out the rest of rotation with rookies and minimum contracts. That's why I think Salmons for 1 year at a slightly higher salary than Mason would require is a good option to fill a slot in the wing rotation next year.
I just don't see much downside risk to acquiring Thomas and Salmons for the expiring contracts. I definitely wouldn't give up Splitter's rights to make it happen. I probably wouldn't give up the 2010 first rounder either, but I don't know enough about the depth of this year's draft to be sure.