Cinepaint is a free app that does exr, if editing that could be a solution. I don't know about the clouds problem and I have never tried a scene like this but the way I look it is that the sun disk is really just a fake thing. The light is still streaming from one point so increasing the sun disk does not increase the area that the light is coming from hence you won't get a glowing ball. There is also a problem with the sun disk not being actually in line with the light source when you move away from position 0,0,0.