Allison, that is a nice greenhouse! I've not seen that one before. I know at one point I had asked about the acrylic type. Is that what that is? I was told something along the lines that those were meant to be more "pretty" as opposed to good for overwintering. The reason for that, is that the acrylic does not have any dead air space which is insulating. I would think you solved that issue with the bubble wrap. They are still great for getting a jump on the season or extending the season.
My greenhouse is only 4 mm polycarb which still isn't great. Normally 6mm is the minimum you would want. Even better is 2 layers of plastic with a fan blowing between.
I had two of the 8.5 x 12 greenhouses that I've now joined to make one larger one. My biggest gripe about the smaller sizes is exactly the problem you experienced. A small heater isn't really enough and a large one is overkill. I would not entertain the thought of trying to over winter ANYTHING in mine. I might try trees that are hardy to a zone warmer but that is about as far as I'd push it. Otherwise, I'd go broke paying for electric and/or gas.
A fan is definitely helpful for circulating air. It's easy for the air in the greenhouse to go stagnant which increases problems for the plants and soil as they are just sitting.
I figure this is long and might cross someone's eyes so I'll end this one. LOL
