MDX was VFR FFS. On what basis did S1 presume to know the actual weather where MDX was going?
Yes, MDX was VFR, which is the whole point. The pilot made a conscious decision to continue flight his flight into known/forecast non-VMC.
Whilst not privy to the information at hand I presume S1 advised that his airspace was "NOT Night VMC" based on Area Forecast and/or Pilot Reports.
And correct me if I'm wrong, the rules at the time were such that if Military Airspace abutted Civil Airspace (that is, contiguous airspace) then the clearance through one constituted a clearance through the other. Therefore if Willy had issued a clearance to MDX then MSX would have had to descend to remain clear of the contiguous airspace.
Anyways, this is all probably just a rehash of what was discussed at the time of the accident. Discussed at the aero club, discussed within RAAF, discussed within SY ATC, discussed within SY FS. We are now 30 years later, 20-20 hindsight, availability of the internet to have these discussions. Unfortunately none of this changes the outcome, the tragic loss of life when the pilot made a conscious decision to continue flight into known/forecast non-VMC.