The reply I received was "the main mode of deceleration is still braking, in general, when the handle is pulled to the maximum thrust level, the engine delay response time is enough for the nose wheel to touch the ground", "perhaps the specific regulations vary by airline".
1. Except in an emergency you don't use maximum reverse thrust IRL. Usually idle reverse is sufficient.
IRL, the autobrake system uses a specific deceleration value. More reverse thrust means less wheel braking. Using more that idle reverse thrust doesn't decrease the landing distance. There's certainly no need to rush the application of reverse thrust.
2. The reason why you wait until the nosewheel is firmly on the ground is that you need a lot of rudder and to counteract the swing if the engines spool up unevenly, even with the nosewheel on the ground.
On the 767 and 777 it's in the 'real' sim almost impossible to stay within the runway boundaries if an engine failure occurs between 30 an 60kts during takeoff.
Once you have experienced this scenario, it's common sense, not to apply reverse thrust with the nose still in the air.
Calculating reverse thrust with an 'expected' spool up delay doesn't make sense, because the engines start to spool up immediately.
Besides controlling the derotation, you would have to very carefully watch the N1/EPR gauges for any delevoping asymmetry. Furthermore derotation time varies a lot with touchdown pitch attitude and derotation technique.