Not sure how well this works, but..
1. Throw em in a ziplock bag with a wet sponge, or even a decent-sized piece of citrus fruit if you don't have a sponge lying around. Anything solid with high water content, basically.
2. Zap em in the microwave on defrost (not full-power! important!) for 5 minutes or so. That'll open up the urethane to moisture, basically.
3. As soon as they're done, throw em in the freezer for a few hours. Take em out and they should be noticeably softer.
You had the right idea with the extreme heat, but they basically need to be steamed or else they'll just melt rather than getting softer.