Firstly, on the five/six, the umpires made their decision after discussion on field. That’s that. The decision was made ande binding.
However, if you are interested, the ambiguity arises from Law 19.8, titled "Overthrow or wilful act of fielder”
“If the boundary results from an overthrow or from the wilful act of a fielder, the runs scored shall be
any runs for penalties awarded to either side
and the allowance for the boundary
and the runs completed by the batsmen, together with the run in progress if they had
already crossed at the instant of the throw or act.”
This is copied from the MMC website. The Laws may have changed since Mike last played.
The footage shows that, at the moment the ball was thrown (ie released) by the New Zealand fielder, Stokes and Rashid had not yet crossed for their second run.
However, is the throw the defining point, or is it the subsequent act (ie the striking of the ball on Stokes' bat), at which point they had actually crossed. So the boundary 4 is not in question, but whether the runs actually run were 2 or one is.
The umpire who has said (on ESPN, picked up elsewhere) he thinks the on field umpires were wrong is Simon Taufel. Good umpire, now on the MCC's laws sub-committee. But it whereas the correcteness of the on field decision isn’t clear cut, once the decision was made then it is final.
So now you know. Or at least you know my understanding.
And the method of determination of the tied super over was in the rules of play since the start of the competition, to cover a very unlikely outcome. That is all there is to it.