1. If the texture is created compress, why would you try to fill it with compressed data ? afaik glTexSubImage* does not do format conversion. The SubImage functions are use to update portion of the already created texture.
2. What do you mean treating it as linear during sampling ? Do you mean the return result or the its treating the content of the texture as linear ? Are you gamma correcting the output being written by your fragment shader? Or are you using sRGB frame buffers?
wrt to 4 and 5, in #4 you said your are certain your GPU does not support ETC2/EAC textures in HW, so I don't understand why you would expect creating an ETC2/EAC texture to not give you an error/