On easy certain treasures such as the ones in the maze cannot be accessed so is it fine to run on easy or would it have to be normal at least?

I thought the maze gives red gem? The left doors before going to fight mendez with the chainsaw ladies have a red gem too?

This is a good point I didn't consider. For Major Treasures you could just get alternative ones like Ben said (unless there are others without duplicates I'm not thinking of?) but for All Treasures you would just be getting all that are available based on the difficulty.