The bug was basically just caused by a missing 'edit_image' and
'edit_image_user' pointer in context for
'node_shader_buts_tex_environment_ex'.
So adding the following there would be enough to fix the bug:
uiLayoutSetContextPointer(layout, "edit_image", &imaptr);
uiLayoutSetContextPointer(layout, "edit_image_user", &iuserptr);

However, I would suggest using the full-flegged uiTemplateImage (just as
'node_shader_buts_tex_image_ex' does -- instead of a "handmade" subset)
for the following consistency reasons:

• Layout was using single column for image textures, but not environment

textures

• Save / Discard feature on editing the image was there for image

textures, but not environment textures

• Environment textures: Color Space was displayed on node (but not

properties)

• Environment textures: Animation / Sequence settings were displayed on

node (but not properties)

Cant think of a reason for _not_ displaying the whole set for
environment textures (just as for regular image textures)?