I concur that "real" OMS cannot attack, corrode or stress crack any steel or other metal parts no matter how long you let them soak. It can, of course, very thoroughly degrease and de-lube a chain so a good relubing is going to be needed after a prolonged soak. That's not your problem.
The "Mineral Spirits" you used are, as cyccoommute noted, an emulsoion of the real thing in water stabilized with a suitable surfactant. Apparently it is not friendly to chains if left in contact for a long time. If you found cracks in linlk other than just the quick link, that assures its damaging properties.