It’s not quite so straightforward. If I recall my history correctly there were several opportunities for the North and South to work something out but to a large degree Syngman Rhee blocked it because he insisted on his own dictatorship rather than devolving any power to various labor-run initiatives around the country. (He was on the US/UN side and still turned out to be a maniac so those UN elections were mostly a symbol) That, and killing a bunch of unionizers of course. It is true the Soviets had their own outcome they were angling for, but without US support Rhee would probably have been forced out and a compromise could have been reached.
Broadly speaking the Soviets were a lot less interested in securing outright puppet regimes than the US. History shows they were more reactive than proactive, so had the US backed off they might well have left it alone.
So we’re talking a bit past each other then. You have mentioned the cause of a divided Korea, which I largely agree with. (Although the US/UN were perhaps too quick on the trigger to hold elections only in the South, though it likely made no difference to the ultimate outcome) I was talking about the cause of the war. In my mind they are not the same thing, albeit two steps on the same dire path.