To me, many cloudy days in a row, with 20-30 degree Fahrenheit temps and a thermostat set at a minimum temp of 60 degrees or 55 as Ray states above is inviting bacterial and/or fungal issues.
Not at all, unless you are sloppy with your watering and cleanliness. There is nothing inherently risky about a plant being at its minimum temperature for days at a time, as the weather dictates. You won’t want to water then anyway, or you will leave pockets of water standing around that
will invite infections.
I’ve also been of the belief that many orchid species need a cooler night temp.
First, that contradicts your last statement. Second, every greenhouse I have ever had, no matter the size, has had several “zones” of different temperatures. In my 55F greenhouse in PA, even with fans running constantly, I had areas that were cooler - lower, nearer the glazing or door, etc. Once the greenhouse is operational and full of plants, you’ll likely see something similar.
I understand sunshine will warm the house in the daytime…but there are many days the sun doesn’t shine AND is low on the horizon.
That is true, but so what? With low light levels and relatively low temperatures, the plants will grow very, very slowly, if at all. If you boost the temperature artificially without there being sufficient light, you’ll force the growth of weak, leggy plants. If your climate is such that you will have a lot of those days, you may need to provide supplemental lighting. Warming the plants is not a substitute!