Yes, you should NOT be Load'ing "GLcore". If you try this, you'll either get the libGLcore.so file from DRI, which is wrong, or you won't get any GLcore at all, which will be no different. In no case will it ever help.
It looks like your X server is set up in 15-bit color. Is this right? AFAIK you have to be using 16 or 24 bit color to have an "RGB GLX visual". Try it and see.
Edit: Just looked at your log file. This does seem to be the problem:
(WW) NVIDIA(0): OpenGL is only supported in depths 16 and 24