Iefan - tube failure mitigation = abort to above 500 ft agl or higher (for obstacle avoidance) rectify problem (if possible) and either resume or abort or continue as you did before you had NVG.
We are allowed to reversionary night fly (ie non-NVG) VFR in UK (military) at or above 500' msd (minimum separation distance).
That leads onto weather limits for the mission - our training limits are different depending on whether or not we have an IFR recovery option - 400' if so, 700' if not - we effectively don't have weather minima for SARops but we have 2 pilot IFR capability. You might want to specify minimum light levels and ensure you have access to met information that provides forecast light levels. NVG is easy on a bright night but down at 1 millilux and below it is a completely different story.
IIMC - again we come back to weather limits for NVG flight - if you set sensible ones and actually stick to them you shouldn't have a problem. if your pilots are prepared to take extra risks to 'save lives' then you will probably end up in the same situation as US HEMS - ie with a poor safety record.
The best option would be for 2 pilots, both instrument rated and a helicopter with a good IFR/icing clearance. Then add sensible operating limits and remove commercial pressure and you will have a pretty safe operation. However, all that comes at a cost so inevitably there will be compromises made.
Since you are operating HEMS and single pilot, I would suggest 700' cloudbase and 3 Kms vis with a minimum of 2 millilux.
Out of interest, we are using XR5 tubes which are an improvement both in low light levels and in high gain situations over the Gen III tubes. Also we don't use visors with NVG due to distortion and misting issues.