Not being able to backpedal on both of the large cogs may mean the chainline is a bit off. Unless you have already checked the actual chainline measurement that would be your first step. If the bike frame was new I would hope that the rear triangle alignment is OK. Moving the crank inward should be OK as it sounds like you are unlikely to use the small chainring with the smaller front cogs. But being able to backpedal more is not a great reason for improving/changing the chainline.

Back when I was a kid there were only two reasons to backpedal - braking or showing off that you had a fancy English racer (3 speed). Now there's really no function at all in doing so.