You need to do some engineering calculations. What will the air temperature in the attic be, compared to the temperature rise available from the sun's radiant heat?Yes the copper would be better but it's so much more expensive and harder to work with. Using pex you can get an unlimited length and work with it rather easily simply using shark bite push to fit couplings. The whole job can be done in less than 2 hours. Running copper would require a lot of measuring and cutting but still I agree the copper would be better. And no I would not run the pex out in the sunlight. I have an enormous attic.with 45° pitch sides making the total surface area exposed to sunlight roughly 166% of the deck area. It is a massive heat machine. Running 100 ft of pex ln a coil through this environment makes for a rather efficient heating process prior to entry into the water heater. There's no pumps I'm moving parts just city water pressure doing the job.
But what am I doing making suggestions as a lay person, to someone who has proclaimed to be an engineer?
Let this one go! The holes in the theory are wide enough to drive an 18 wheeler through.
Fwiw, your 6/12 pitch in the attic doesn't make a goddamn bit of difference. You're trying to rescue your original idea by inventing a hybrid!