Ok, here's the first attempt. It took over 24 hours, but that's because I raised the quality of the halo atmosphere shaders to 1 to get the desired smooth render results. I think I can lower the atmosphere sample setting a little bit next time (currently at 24), or perhaps reduce one of the render node settings like AA or detail by a point. I also reintroduced some transparency to the water shader, which in retrospect was pointless for a low-light render like this.
Any comments on how to improve the quality and/or realism of the render are welcome, but what I can't figure out is why the 4 halos turned out different. The ones on the left have a distinct cutoff to the haze/glow effect, while the ones to the right are pretty close to the effect I'm after, a nice falloff with hardly any detectable margin. I checked all four light sources and halo shaders three times to make sure all the settings were the same. I think it has something to do with the fact that the two on the left are farther away, which might affect how much the density of the localized cloud has on the light source.
You also might notice a strange dark border slanting horizontally through the middle of the render. I'm pretty sure that's due to a shadow effect of the terrain, as the sun is very low on the horizon, 2 degrees. I think I can correct that by raising or lowering the angle just a trifle.
