Answer to queston 3: Depending on the scene's ground plane.  If you use a plane mesh which has mountains, dunes, and mounds then the dome would be most satisfactory for this particular scene strictly because plane height would hide the ground and dome seam.  If you are using flatter terrain then the hemisphere would be more appropriate.  Using the entire sphere would involve utilizing an image which may contain a 360 degree ground and sky or predominantly sky or interior.

Answer to question 4: To still use a narrower focal length (i.e., 75mm) you would require a very high image resolution which I have forgotten atm.  However, these high resolutions can bog down your system and make Poser slow to respond.  Decreasing the focal length will widen your field of view or incorporate 'more' background (viewing more pixels) into the scene sharping the image. 

Answer to question 6: If your interior scene incorporates windows, a dome will add more realism to the render.  However, ambient value must be increased to a rather large value for IDL to illuminate interiors correctly and realistically.  500% or higher sounds about right.  Now, if your generation lighting from fixtures, candles or torches, inverse square point lights would be adequate.