However Boeing also advises us to bleed all of the headwind additives off over the threshold
Well that's not quite correct, you know. The FCTM wording is: "the gust correction should be maintained to touch-down while the steady headwind correction should be bled off as the airplane approaches touch-down." There is no way you can deliberately bleed off (say) 15 knots - assuming headwind component of 30 knots - between threshold and touch down unless you deliberately prolong the hold off to do so. And that's not on for many well published reasons
The reason behind the HW additive is because of surface friction effect on wind. The windspeed drop off caused by surface friction ceases at around 2000 feet where it becomes known as free stream flow. This means that below 2000 ft (assuming no significant terrain) the pilot should commence the gradual speed reduction in order to arrive at the flare at Vref (plus any gust factor). Of course you would brief the PNF of your intentions in order to avoid the scream of "Speeeed' from the keen and enthusiastic PNF.
See FCTM page 6.11 under sub heading Normal Touchdown Attitude which states: "It shows the airplane attitude at normal touchdown speed for Flaps 30 (Vref30 to Vref 30 minus five knots). With proper airspeed control and thrust management, touch down occurs at no less than Vref minus five.
Providing the correct flare technique is used the aircraft will only lose three knots during the flare manoeuvre. Boeing avoid defining the term "approaching touchdown" with some arguing that short final is approaching touch down after a 1000 mile flight..