You can cool via venting only. However that requires a large volume of air exchange. Something on the order of replacing the entire greenhouse air volume every minute or two. Greenhouse get very hot very fast in the summer when the sun is out!! Also keep in mind that without evaporative cooling your inside air temperature will at best be equal to the outside air temperature. In practice it will run 5-10degrees higher depending how fast you vent. With evaporative cooling it is possible to run the greenhouse inside air temperature LOWER than outside air temperature. While at the same time reducing humidity drop or if desired increasing it above outdoor levels.
Personally I wouldn't bother with a triple wall 10mm. I think you will find the r factor only slightly better than the double wall 8mm, and noticeably less than the triple wall 16mm. The r value is related to the number of layers but more significantly the thickness of the air pocket.
You can design a greenhouse to run at whatever conditions you want on a year round basis. The deciding factor however will be.. can you afford the energy expenses! If your wanting to run lowland conditions in a subfreezing climate then you would need a really well insulated structure. A big wallet for the heating bills. I have been considering something to grow lowland plants in and it might come down to building my own with polycarbonate 8mm walls covered by an inflated 2 layer poly.
As for winter power failures. They are an ulcer maker.. so are summer power failures! In the past I had one of each.. a winter freeze and a summer roast. That was a hobby greenhouse with just one heater and one set of circuit breakers. In one of the cases the power didnt go out but the breaker tripped.. which has the same effect when everything shuts down ;/
Now I have my greenhouse(s) set up with an automatic generator. TWO heaters TWO vent fans Everything on it's OWN circuit breaker. Probably excessive for a hobby situation but there other ways to avoid the ulcers. Alarm systems are great. They can be as simple as one of those wireless temperature sensors so you can keep an eye on things to ones that will ring a firebell if the temperature gets too hot or cold beyond whatever setpoint is set. They even make little computer gadgets that monitor a greenhouse environment and will make phonecalls to a couple preset numbers if temperatures go outside preset setpoints. You can even call them up and get a message about the current conditions. They of course require a phoneline within the greenhouse.