The concept has a lot of potential with high stakes and a clear danger that needs little explanation. I believe that with the exception of M.A.S.H there have been few main stream successful films made about the Korean war so the subject mater could prove to be an interesting addition to the war genre.
The logline construct is a problem as it doesn't read well due to lack of grammatical refining and well described plot points.? What is the starting point of the story? What propelled the MC into action? What is the inciting incident?
Could it be the death of the commanding officer which was leading the fight to keep the route open that made the marksmanship instructor take command?
What is his flaw ? What is going to be the MC's journey of change throughout the story?
It is this that will make this a moving story to watch.
His goal s to keep the evacuation rout open this much is clear but the story cold greatly benefit from an antagonist that is motivated in direct opposition to the MC. Could there be a specific Korean commander you could describe as the antagonist the MC is up against?
As this is an adaptation from book to screen you should feel free to dramatize the conflicts, actions and struggles with visual events. Point is you don't have to stick to the original story page for page because you will likely have to make significant changes to make the story work for cinema.
Hope this helps.