[quote]Radar guidance would have only been available if they had followed the the RNC route down the sound, [/QUOTE
Not correct. Following from Gordon Vette "Impact Erebus"
It is probably correct to say that Collins would have been genuinely surprised while he was still at safety altitude approaching McMurdo to hear McMurdo offer him a radar let down at that altitude.
He probably did not understand their reasons or else he would have been surprised that they had the radar switched to surveillance mode, which, even though it had its blind spots overhead, would allow them to monitor his initial approach and probably allow him to come below MSA on approach.
However, during the next stage of the approach his views would have been altered. Even though he saw that his transponder was being interrogated, he still apparently did not get any radar direction.
That lack of any contact with the radar operator would be because the radar operator only had VHF, if the flight had of continued on the track it was on above MSA then the radar and VHF contact would have been made, and a DME lockon achieved. Instead, for some reason he requested a VMC descent.