But the war and the planning it required helped the American economy, it wasn't a waste. Without entering the war the American economy would be smaller in 1945 than in OTL. In the OP's scenario, the Soviets also have the resources and, more importantly, the experts of continental Europe at their disposal. They can turn to handle Japan at their leisure and secure communist dominance in Korea and China. Although an invasion of Japanese mainland is unlikely, the Soviets don't need to cross an ocean to threaten it like the Americans did.
Sure, Europe won't rebuild as fast without the Marshal Plan, but plenty of countries didn't get the aid IOTL and they turned out just fine. The real questions to be asked are more political than economic: Would the Free French recognise the new Soviet backed government in Paris? For how long would isolationism be dominant in the US as long as nobody attacks it? What would the UK's position be towards decolonisation and the new order in Europe.
The US was already recovering before entering the war. The US Spent ~40% of its output on non-productive things (weapons) during that period, spending some of that money (the US had already massively increased its military budget in 1940 and 41) on productive things, infrastructure, investment, trade, would increase the economy more, plus the US will still be selling weapons, resources and supplies to anyone with the cash to pay for it, as the OP said no embargo
The Soviets also spent two years longer at war in the OP's scenario and did not get lend lease, two more years of delayed investment and maintenance in infrastructure, two years more of young men dying en masse (my guess 3 million extra, plus several million civilians), I'd be surprised if they could hold down Eastern Europe to the same degree as OTL, much less Western Europe. It certainly won't have the GDP of OTL
The US had abandoned isolationism before Pearl Harbor, when France fell 88% of the US supported arming to the teeth (which it started doing) and 71% universal conscription, before Pearl Harbor 70% of the US was in favor of going to war if needed. If the USSR takes over Western Europe, well the US is going to take that seriously
Free French are unlikely to recognize a Soviet Government, though IMO it's more likely they'd just waltz in after the Soviets take Berlin/Rome and the Germans and Italians pull everything out to stop them