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.
Europe can be warmed by even the dried shit of Ukrainian refugees, if it is sufficiently democratic and carbon neutral. Only 10-15% of the gas consumed by Europe is used for heating, the main consumer of gas is industry, and fertilizers for agriculture are also made from gas. The rejection of Russian gas will cause the collapse of the entire energy-intensive industry in Germany and Italy.