You should be able to pull the emergency release and open the garage door manually. The spring boost means it’s not that heavy.
Also, like you we use space heaters. The sealed “oil-filled radiator” type. I have like 7, but only 3 or 4 are in use at any given time. Only use for the furnace is a quick warmup in the morning, but we would forego that during a power outage. Also, the basement stays cooler and warmer than upstairs depending on the season.
That is the theory. Before you count on it see if your wife can do it at 2 AM with the temperature in the teens and you are having a medical emergency...