one example being why not only use the new method when LHR is busy as opposed to slavishly sticky by it all day
It's presumably the fact that the trials have been operating pretty well continuously since August that has led to the recent announcement by the airport that they now have enough data to allow their early ending.
Restricting the RNAV departures to certain times of day would have required a longer duration to gather the same amount of evidence.
With hindsight, that might have been a good subject for a consultation before the trial was designed.