1. Absolutely! You can use a 3D modelling program like Bryce to make skies, but the easiest way to make a planetary skybox is to use TerraGen. Here's a tutorial
I wrote on the subject.
2. Ahhh...use q3map2
no matter what, it far outstrips the compiler included with JK2Radiant. Now...if you do a "nolight" compile, then everything will be fullbright, since the sompiler doesn't take the time to calculate what light goes where. If you do one of the other compiles and the light process is supposed to be run, then the reason you're seeing the "fullbright" error is that there is a LEAK in your map (the leak will cause the compile to crash out of the VIS phase and never execute the LIGHT phase...so it's just like running a nolight compile). Load up Gtk and click File > Load Pointfile. You should see a big red line running through your map. The line originates at an entity, and will go through the map until it goes through the leak spot. Find the leak and plug it up. Remember, entities, detail brushes, and patches won't seal off the void, so hit ALT+2, CTRL+D, and CTRL+P before hunting the leak.