...so is it not possible that perhaps they in the heat of the moment had managed to tune the wrong frequency?
No, I don't think so, but it would depend on the operator's SOPs, whether VHF#2 had been used to monitor 121.5 during cruise, and whether it remained on that frequency during the descent and approaches.
Is that not a plausible explanation for the missing Mayday call?
Yes! That is not a
plausible explanation.
But, why don't we leave this for the ATSB to work out boeingbender?