It seems point 2 is the culprit then. That would explain bot the lightsaber blade visual glitch and the unusual shader rendering errors. The lightsaber blades are using OpenGL to render.
Update your graphics drivers, @, and switch to the latest version of OpenJK, as @@Xycaleth recommends. You may also need to remove/disable GLDirect.
Edit:
Sorry, I missed the last page of the convo, but yeah, try updating the graphics drivers, and see if that helps with the rendering.
I have 8.1 as well, and it runs perfectly fine here without DirectGL. It definitely has to do with your graphics card. Driver updates!