I can't say with certainty, but SCCo Board of Sups would make the decision. There might be some inclination to put it to the masses under an advisory vote if the Sups are feeling like they might be putting their political futures on the line but no legal requirement. If a stadium was to be built on the fairgrounds property, for all intents and purposes, it would require the approval of the City of SJ council. First, there is the issue of the City of SJ influencing what SCCo can do with the land by tying them up in court like they are doing with respect to "battling concert venues" (despicable). Second, there would be quite a bit of infrastructure improvements that would be needed to make a stadium at that site successful (roadways, sewer, etc.). The site is totally surrounded by the City of SJ. Yes, unless you are the one owner who controls the league. I had some input on the subject here.
No, I don't think it would be beneficial to the league as it is set up now, but it sure as hell would be beneficial to the game. I have a very strong feeling that if each team in the league was owned by a separate owner, it would bring some dramatic changes to the way the league is run. Each owner (i/o, whatever) would be more protective of its interests and wouldn't allow any other i/o to pull off the kind of chicanery we've seen.