MEL: wot he on high said!
If it is a part of the minimum equipment list for the operation, then it must be serviceable before embarking on that operation.
Moving map is a minimum serviceability item for us to do our rapid response HEMS. And as per the outcomes of the Norweigian analysis, it should become a minimum requirement for HEMS into the future.
We actually teach a mix of NVG and instrument scans, indeed most of the work during NVG flight is substantiated on the clocks. We teach that the primary closure rate aid is the airspeed/groundspeed indications and the primary descent rate info is the VSI until you get to around 400ft (or lower dependant upon illum), then you can gain primary info through the goggles and secondary back up using the instruments and crewman con.
Popping down off the instrument approach on NVG is not confusing. If at the minima you are not visual, go around. We dont permit the "scud running" you are alluding to when on NVG as discussed before in your example of the inter hospital.
There is no way that a chart or forecast can determine the scene illumination due to the enormous variables previously discussed. Accordingly, unlike almost any other form of flight, NVG can be a "go and have a look" operation sometimes. It also means that the only possible way to achieve a minimum illum condition is to trust pilot judgement of that distance.
Scarry concept - trusting pilot judgement!
That said, on our organisation we have the crewmen who are repeatedly trained in assertive controls when faced with a deteriorating viz situation. They are trained to estimate the 5000m and play a critical role in enforcing it - that way there is two of them making the call and reducing (not eliminating) the chances of temptations to ignore the situation.
The Oz regs actually borrowed a procedure from the EASA proposals that the US do not use - the "turn back" procedure in response to deteriorating viz/illum/ground definition; either crewmember can call it, the acft should them conduct a level decellerating turn slowing to about 60 to 70 kias until regaining required viz.