"- is it possible to render out 12 fps and then use some post-processing to convert it into a smooth non-blurry 24 fps animation? Is there goos software for such interpolation? (What about 8fps to 24fps conversion)"

http://www.thebest3d.com/howler/7/motion-prediction-module.html

Motion Prediction in Dogwaffle does that. It can produce a noticeable framing effect around the edges, and they recommend rendering larger than you need, and cropping the edges out. Otherwise, it looks pretty good, and is very fast.