Hey man,if a hps light isn't keeping that space warm enough then there is definate some leaks/drafts.Insulate with stonewool or kingspan then cover with black white/mylar.If this isn't enough bang another light in.More bud anyways.Or 2 x 400's instead of a six.Metal halides run about 10% hotter than their hps equivalent.Deffo pay for a light rather than a heater,the light will heat and pay ya back.
If u have an intake you could get a duct heater,a filiment that fits inside of the ductwork.Or pull your outake through a filter and send it back in the room again,so not losing heat.
I think the other farmers amd yourself are bang on,they are only gonna produce normal amounts of bud with a normal enviro,with the stress and changes they have had they prolly just slowed right down,but once they are back in swing i reckon you'll still pull something worthwhile,i seen a mates plant[online] attacked by mites,looking like all was lost but they have an amazing abiltity to heal and bring themsleves round so plenty of TLC and you'll be right.They just need a consistent enviroment now till the end of the run.