Did America help the Chinese nationalists to repel the communist advance?

Did America participate in China's Civil War (1945-9), to repel the Communists?

No, they did not. Marhsall's role was to intermediate between Chiang Kaishek's Nationalists and Mao's Communist, to avert a civil war. Not to start one, nor to use force against the Communists. Truman was blamed during later stages of the Civil War (1945-9), when it going the way of the Communists and Marshall, frustrated with failure to achieve his diplomatic mission, pulled out early 1947.

Why didn't the US government get involved in the war?

Too many reasons but it boils down to politics (no popular support for yet another war, after WW2) and lack of intelligence (or understanding) of Mao's Communist Army. This and a lot more details are available in "The China White Paper" (1949) by Acheson, Truman's Secretary of State. This "loss of China" to America, which supported the Nationalists, contributed significantly to the rise of McCarthyism.

Although it is nearly impossible to prove a negative, a logical way of deducing this would be to check Wikipedia's list of battles in the Chinese Civil War. Only the Nationalist Revoltionary Army (NLA) and th People's Liberation Army (PLA) are listed as belligerents. Not the US Army.

Additional What-If Questions

History tries to answer one (hopefully, logical) question at a time. Speculation is beyond its scope. I am not a historian, nor am I good at the speculative question you've asked.

Instead, I recommend that you try "China 1945" (Penguin, 2015) by Richard Bernstein, who studied history under John K Fairbank (one of America's most promienent historian of China). Bernstein wrote this book not as an historian, but from perspective of a journalist (at NYT). This book has lots of "What-If's" on the Civil War. NYT's review.

For what it's worth, your additional questions does not surprise me, because the "loss of China" is still being considered today, "Could Truman Have Worked With Mao?" (2017). An historian and above-named Bernstein participated here.

Deploying vs Employing Force: China & UNPROFOR

Finally, regarding the 50,000 or so American troops in China -- during war or military operational conduct, a distinction should be made between forces deployed and forces employed. One does not necessarily lead to the other. In other words, deploying military forces to a theater of war does not necessarily mean such forces will be used.

I will quote General Rupert Smith, the UNPROFOR commander in Bosnia and Herzegovinia (1992-1995), during Yugoslavia's Civil War (1991-2001) (emphasis mine):

"Industrial wars had clear-cut strategic goals. It has been used to create states, destroy the evil of fascism and end the Ottoman Empire. In war amongst the people, however, the ends to which we use military force are changing to something more complex and less strategic. ... (If) there was no strategic side to conquer: the enemy by design often did not present a target that was susceptible to strategic attack, since in most conflicts of war amongst the people the enemy is in small groups operating at the tactical level, against which the manouevres and mass firepower of industrial war are ineffective ... (This) mean that, much as the political aims have changed, so has the use of force: the conflict are fought for sub-strategic objectives.

The term "sub-strategic" arises form the confusion between deployment and employment. We may deploy forces strategically in terms of distance or in teams of the level at which the decision to do so is made. ... However, none of this indicates the level or purpose to which is to be employed."

Source: The Utility of Force: The Art of War in the Modern World (Penguin, 2008), pp. 272-3.

He was referring to "peacekeeping forces" of late 20th century, with which he had direct experience, but I hope you can see the similarity of sub-strategic objectives that the American forces had to deal with in China, mid-20th century.