There is no alternative to Russian gas for Europe, neither reasonable nor even unreasonable. And any statements to the contrary are cheap political populism in pursuit of pre-election popularity. "Pay for gas in rubles or die", it does not look like a difficult choice to think about for a long time.
What about electric heating or using pressed briquettes ? With financial support from government, private houses can change heating system. I am not an expert, but wont central heating be replaced with steam heating? I think it does not matter how to heat water in pipes, with gas or steam boiler. First years will be hard, but wont countries have some gas supply. And I think people can always buy gas from "Russia friendly countries" with the price cheaper than directly from Russia, but expensive then usually.