Efficiency of soft solvents like ethanol, rubbing alcohol, whitespirit and others depends on type of primer you have used. But I'm in doubt that they'll help if you say that Simple Green did nothing.
I'd recommend braking fluid (DOT4 or DOT3, the second one being better). It won't harm the plastic besides bleaching it a bit. It can soften the plastic, but don't be afraid - miniature would harden again when fluid evaporates. It will dissolve any acrylic/nitrous hobby/automotive primer I ever know.
Don't waste your time on "soft" solvents. Don't buy whole can of braking fluid. You only need 100-150 cc. Try to find a garage, even used braking fluid will work well. Work in a good ventilated area or better outdoors. Sink miniature for 15-20 minutes, rub it with toothbrush. Rinse and repeat until you're satisfied. Don't apply force, better repeat the cycle with longer sinking time. Be warned that braking fluid should be discharged properly, local laws and responsibility wary from place to place.
May the 4th be with you!