Hi Cracka,
From my arguments with Mazda about load carrying, I don't think they consider practical application of their statistics. 350kg on the towball will still be within their specs for maximum weight on the rear axle (1400kg). Towing with 350kg towball weight and not exceeding max rear axle weight is not their problem.
I put 900 kg in the tray of the Freestyle HiRider, pushed up against the cab as much as practically possible, having minimal weight rear of the axle and rear axle weight still came in at 1550kg. I argued that the ute could not carry the 1450kg payload advertised and reinforced by their salesman. The service manager replied that the ute would carry the weight if the weight was evenly distributed across all four wheels. I asked him to demonstrate how that could be achieved and said, "It's not Mazda's place to tell you how to pack your vehicle."
So back to "Is it safe to drive with 350 kg towball weight and no other weight in the tray" If 950 kg in the front of the tray exceeds the rear axle weight, then 350kg on the towball will take the rear axle weight to its maximum, leaving you with no ability to carry extra to stabilize to ute. I wonder if Mazda specify if it is safe to drive at speeds above 80kph with 350kg towball weight?
The devil is in the detail. These are maximum weights and should not be taken as daily practice without modifying the suspension.
Cheers
Murray