With a POD after 1948, Have the DPRK consist of Korea, Gando (A disputed territory between China and Korea), and Priamurye. Somewhat like a greater Korea.
North Korean troops successfully break the Pusan Perimeter. Inchon landing then proves to be a failure. War ends with communist control of the entire peninsula.
Later on, the Sino-Soviet border war breaks out into something a lot bigger, and Korea sides with Moscow. Korean troops successfully push into and seize Gando while the main PLA forces are engaged with Soviet troops further north. The war ends a few months later with the line of control becoming the new state borders.
I don't see a way for Korea to take control of Outer Manchuria. Any attempts are likely to be repulsed, and if Korea does take the upper hand, a lot of their peninsula would soon glow in the dark.
Probably they'll get Outer Manchuria after the USSR dissolves?
Dewey wins, and doesn't intervene. Or is too late. Cultural Revolution leads to unstable China which starts WWIII.
Avoiding WWIII is easy if you don't care about Outer Manchuria though.
Dewey was an internationalist by that point, and, IIRC, more focused on Asia then Truman was. I think Dewey would intervene as well.