Have you called
Britax customer service? I'm guessing the problem you're faced with is that the vehicle headrest pushes the top of the
Marathon forward so that the back of the carseat is not flush against the vehicle seatback cushion
Britax may say this is fine so long as you're following both instruction manuals to achieve less than 1" movement at the belt path ... however, they may not (in which case the
Marathon would simply be incompatible with this vehicle when used
FF)

It's much easier to help solve situations like this in person: have you tried contacting a local CPST?