The fundamental question is what you want the greenhouse to do. In my super short season, adding 36 days at the beginning gives me 20% more season to put to a good use. Not only that it should allow me to bring up 2nd year plants to market size by early June. Also the plants I work with are as winter hardy as they come; if I keep them a tad above freezing at night they will still grow. This all means that because of the short period I need to cover I can afford to pump some heat into the greenhouse, with a eye on pumping it into a thermal mass to regulate temperatures at a level lower than most people think about.
The newer blankets are more water and weather proof. But maybe not PNW proof. And the PNW has greenhouse issues, namely not enough light and heat in winter. Even the Chinese method doesn’t work well if the sun never shines. It would ameliorate your occasional 20F freezes. But so does a cheap heater.
I’m venting heat in my GH most days all winter. That’s because we have 75% sun in winter and minimum 10.5 hr daylight vs 8hrs and 20% sun in PNW.
If I were to try to use a rootstock for dwarfing figs I would try grafting on to ‘Little Miss Figgy’, it one of the most dwarfing fig varieties, and it’s easy to get, although I don’t know how much of it’s dwarfing effect gets passed on to the grafted on part.