Question: space1_bk, space1_dn, space1_ft, space1_lf, space1_rt textures are exactly the same, could we keep only one and refer to that one with shaders?
don't think that's possible. this is a snippet from the q3a shader manual describing the farbox in a skybox:
3.1 skyParms <farbox> <cloudheight> <nearbox>
<Farbox> Specifies a set of files to use as an environment box behind all cloud layers. Specify "-" for no farbox, or a file base name.
A base name of "env/test" would look for files "env/test_rt.tga", "env/test_lf.tga", "env/test_ft.tga", "env/test_bk.tga",
"env/test_up.tga", "env/test_dn.tga" to use as the right / left / front / back / up / down sides.
as i understand you need all these files. imo it's not worth the effort to go around this just to save up on a couple of images.