Difficulty in getting the rd to shift into biggest gears, and issue with backpedaling issues really suggest bent RD hanger. On a new bike, that is not super uncommon. They have been shipped, and stored, and handled, and that never really gets checked in assembly.

Best means to check it is the tool that bolts into the hanger, and then measures the distance to the wheel as you swing it. You can make one yourself with any solid bar and a drill press.