"...Rather than take depreciation, client prefers to take the full deduction of vehicle (via section 179) in year one"

§179 *IS* depreciation.  Take that in year one, and you are stuck with actual in the first year and going forward for the ownership of the vehicle.  

Take mileage in year one and you get to go back & forth using SL.

I ask my clients for the miles, total & business.  If they say it's 100% business and they have other vehicles - I take them at their word (along with getting it in writing on the organizer of course).