It might cause the PC to overheat. You can paint glass, but PC is not as heat resistant. My polycarbonate installation manual expressly warns against putting shadecloth directly underneath polycarbonate against the plastic, as it will destroy the PC surface from the heat.

Reminds me of the story of Tessie Oelrichs of Rosecliff Mansion in Newport. She wanted to have a white ball...all the flowers, food, everything in white. She wanted her guests to wear white including powdering their hair. And she realized the water was not white, so she contacted the head of the Navy in Newport and asked him to float his fleet with their white sails in the water for her ball. He told her to go pound sand. So instead she hired workers to build and float fake sailboats out in the water for her ball. Amazing. I remember too that the chandeliers had bowls in them which would get filled with perfume so the heat from the lights would cause the fragrance to waft gently down to the guests. If you can imagine at that time, her annual budget for the perfume alone was $65,000! But Rosecliff is a beautiful building, nonetheless...with its sweetheart staircase...

You're over-thinking this detail. If the boards are level enough for modern fiberglass reinforced asphalt shingles they are certainly level enough for Ice & Water Shield underlayment. The major beneficial feature of this material is that it seals the roofing nail holes and seals to itself at the laps. How well it adheres elsewhere is a side benefit that will only matter if you are ever in a hurricane. How much wind uplift resistance do you think stapled double coverage felt underlayment provides? I have a sample of this stuff from the 70's and the two lapped pieces are still stuck together. If you ever put the two sticky sides together you will not be able to separate them without tearing the polyethylene sheets away from the asphalt layers and that is quite difficult to do. You can run this stuff vertically, lapped the wrong way, or forget to remove the backing paper and it will still perform better than any other underlayment. Use the real Ice & Water Shield or make sure any substitute is 40 mils thick with a smooth polyethylene top sheet. This is the same material as Vycor 40 but with a non-slip surface. You still want to be careful walking on it when wet.

Also wanted to ask if cellulose or fiberglass insulation would be a better choice for the attic. I live in a cold climate (Michigan) if that influences the type of insulation.