The first law requires that "all passengers in the front or back seat of the motor vehicle are restrained by a seat belt assembly if between the ages of eight and fourteen" and further states ""Seat belt assembly" means the seat belt assembly that is required to be in the motor vehicle under any federal motor vehicle safety standard issued pursuant to Public Law 89-563, the National Traffic and Motor Vehicle Safety Act of 1966" - so the way the law is written, all passengers ages 8-14 must wear the FMVSS seatbelt, not a harness in a carseat.
The second link I posted makes a distinction between carseats and "seat belt assemblies," so that if you wanted to read "seat belt assembly" in the first law to mean "carseat," you really can't, because the second law draws a distinction that a "seatbelt assembly" does not mean a carseat. Now, the second law specifies that carseats or boosters are the requirement for passengers under 8yrs. (or 57" and 40lbs), but makes no allowance for passengers over 8yrs. to travel restrained by anything other than a seat belt. Seat belt [assembly], not carseat.