Never heard anything from Mahinmi. He has returned to SA in at least one summer to train with Duncan & others so that relationship is good.
He would be close to ideal. Pricey, but should be less than the max and leave room to get guys like Bertans and odds and ends. He was one of the best defensive bigs in the NBA, protects the rim and is handles pick & rolls a lot better than Spurs centers. Can finish strong. Doesn't need the ball. Has relationships on the team & in the city. Knows the system as it used to be and could probably pick up on the changes quicker than any new player would the whole thing.
But ... he's 30 next season and finished this year with back issues that limited him to about one great playoff game. I wouldn't be surprised that if he signed a 4 year deal anywhere that he would only be effective the first two seasons then be a problem as age and injury ruin his athleticism and he no longer guards the pick & roll well, finishes less, and is slower to contest shots. A backup paid like a good starter.
And of course Pop screwed him over pretty hard his last season here. One thing for Pop is that he does seem to maintain good relationships regardless of what he did. Hill still likes him. Bowen has never had a bad word for Pop despite the same not being true in reverse. So maybe Ian doesn't mind.
I do think it might be a 4/60 deal though.
As far as Marvin Williams goes, he's routinely only performed in contract years and been mediocre or worse otherwise. And he's been getting paid like Splitter and will probably want a raise to 10+ annually. No thanks.