“Putin could embargo Russia’s ~~ 8 million barrels per day of oil, and embargo Russia’s exports of 1/3 of Europe’s natural gas.”
As far as natural gas to Europe is concerned he doesn’t have to embargo. He can just raise the price. There is no place the Europeans can go to replace Russian gas in the short to medium term. Pay up or freeze.
You are right - Russia supplies their gas and they must have it. Putin is not sitting there waiting for ruin - he will do something to harm the US (actually can you say “Obama” whom he despises and I don't blame him). Like Palin sort of said, “I can see Russia from my house.” Well, I can see WWIII in the not distant future.
Europe does have gas reserves. So far this winter has been very mild and many of the central heating stations work also with heating oil. Then there are already some LNG terminals.
Russia can not make Europe freeze in the short term with threatening to cut their 30% supply. They also need the money they get from selling this gas and such move from Russia would certainly be retaliated.