Welding the ball to the hitch will not compromise it, so tack welding it will definitely not. Been hauling 10k-12k for 30 years and I slept at a holiday inn express last night !Then you should know that tack welding the nut to the hitch will not penetrate through to the threads, which would compromise the strength of the nut. Similarly, tack welding the ball to the hitch will also not penetrate through to the shank.
Unless you are the god of welding and can tack a full pen weld, I think you are over-complicating things and getting out of your depth.