I am actually in Halifax, NS. Just an interested novice, so this isn't tested advice but (if they have potential for your application) perhaps a more experienced member can assist with the details.
Can you create a second set of plastic transparent windows to create an air pocket between? I have read 3/4" minimum spacing and too much negates the effort. That may decrease your heat loss.
DIY Solar heaters? I'm seen them around the city more often so they must work to some degree. The typical beer can panels. It may not solve your night time winter issues but perhaps in conjunction with the window blankets it may give you some appreciable difference. You may even be able to heat water barrels via solar water heater, and the barrels then give off heat through the night. Seems like a few small scale, inexpensive experiments would give you enough evidence to test it on the greenhouse or not without any serious modifications.
If you can increase your heat will you increase your air circulation with the natural currents of air rising and falling? I pulled this from another site in regards to ventilation: "...greenhouses that are built from more rigid materials need to follow a few guidelines to ensure proper ventilation. The most important aspect of natural ventilation for these greenhouses is the size of the ridge (roof) vents and sidewall vents. It is recommended that the total combined ridge vent area should be equivalent to the total combined area of the sidewall vents and each should equal approximately 20% of the floor area. For example: a 10 ft x 30 ft greenhouse would have a floor area of 300 square feet; 20% of 300 is 60, so a 10 ft x 30 ft greenhouse should have a total area of 60 square feet in ridge vents and 60 square feet in sidewall vents to be properly ventilated naturally. (http://gardenandgreenhouse.net
And something like this example may be of interest:
Best of luck! and interested to see how you make out!