The best way for a leftist France to stay independent of Stalin would be for them to liberate themselves. That's how Tito pulled it off. Even if the Anglo-Americans aren't successfully invading, significant aid would likely still get to the Maquis. If the Soviets are threatening Germany proper, most German forces in France would likely be withdrawn. An uprising in such a vacuum could be successful, and liberate France in time to prevent Soviet troops moving past Germany. Whatever sort of leftists in charge in France would still have a tough time fending off the Stalinists, but if they can maintain significant independent armed forces it could work.