While I can't speak for your SKS, I soaked the parts of my WASR in mineral spirits for over a week and had no ill effects. Parts on that are parked and blued. Pick up a can of brake cleaner too. Attach the straw and use it to blast out the stubborn stuff and make sure the bolt/firing pin is free and clear.So soaking the trigger assembly, magazine assembly and bolt cover in mineral spirits in a bucket for a few days will not harm the black/blued finish?
As for the stock, Formby's Antique Furniture Restorer works very well. I used that on my M1 stock, followed by boiling water, followed by Dawn dish detergent rubbed into the stock and rinsed off with more boiling water. This will raise the grain, so be prepared to do some minor sanding. It'll also raise minor dents and cartouches (do SKSes have cartouches?).