Chris, I'd recommend you try one thing at a time, or you won't know what the problem was.
i.e. If the plant recovers and you gave it both fungicide and magnesium at the same time you won't know which remedy worked so you can prevent it next time.
As far as heat for the greenhouse goes, a large tub (like a plastic storage tub) full of water in the middle of the greenhouse can make a great heat sink. It warms up during the day when the greenhouse is in the sun, and releases its heat slowly through the night. Also helps keep the humidity up. I had a friend in Salt Lake City who heated his greenhouse with just tubs of water and waterbed heating mats to keep the water warm on the coldest nights. Easy, cheap and safe, too!