I have ported your code to a fiddle and cleaned it up a bit. Now it’s easier to work with the example:
https://jsfiddle.net/m7tvxpbs/
Unfortunately, I was not able to manage yet to avoid drawing the scene twice. You can see the problem right now how the render function works. It’s necessary to render the scene once in order to save the depth information. Then you have to render it again together with the soft particles. I think it’s possible to draw the scene once, then the particles and composite both results into a final image. But like I said before, I was not able to do so since the blending did not work so far. At least, we have now a live example so others can play around with the code.