the plan that poser had was to use the environment shpere ligh  to light up the scenes in poser but they changed it and not finished, so now the environment shpere map is the same as the L/L sphere map but it do nothing.. as you said only to match the light and reflection.. so that why i created the new sphere so it match with the all maps and light probes other it make not sense..
you have to plug it only into the Alternate Diffuse and set the Diffuse color to 0.. nothing else.. it can't be affected by the HDR, it can't have any shadows.. and need to be used with shadow catcher..

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"Surrender to what it is - Let go of what was - Have faith in what will be "