Your referenced speeds for the 737 are for an overweight landing at the lowest allowable flap setting. 186 mph for the G450 would also occur at a weight above max landing weight. An incredibly low likelihood.
But we should consider the worst case scenario, not an average one. Thus 230 mph is conservative. They didn't even consider fast gliders, which alone can reach 460 mph.
Do you have a reference for fast gliders? I’ve never heard of anything like that.
I don’t think the FAA plans everything for the worst possible scenario. The odds of a classic 737 landing at max take off weight and thus high speed are infinitesimally small. I don’t have a landing distance chart but at the max over weight landing it likely can’t even land on 99.99% of runways in the US in that configuration.
If the FAA planned and approved everything at worst case scenario then we wouldn’t fly anywhere.