To your questions: #1- I remove seats to keep them clean and dry. Handle bars are above the hood line but not interfering with fwd vision. #2- headlight bean shines through the spokes but there is no noticeable impact to the beam. #3- No change in mpg that I noticed. #4- mounted to the brush guard, so no bumper contact.
Make sure you plug the seat post if you remove the post. Any moisture down that tube and the frame, if steel, will start rusting quickly. Use a product called Frame Saver and spray it liberally down there to prevent corrosion.