Soon after the start of the race when the sled goes over the cliff, the musher throws out a snow-hook which prevents him from falling to his death. Snowhooks are connected to the bridle, not to the musher.
While it may appear in some scenes that the American flag has 50 stars; every flag has the correct number of 48 stars as would be appropriate for the 1917 time period of this film. View time-stamp 1:06 as an example of 48 stars running parallel on a field of blue. Some scenes in the film make it appear as if the stars are on a diagonal, rather than parallel, due to the wind shifts and foreign objects partially blocking the flags.
After Will crashes after dodging the "Army" truck. It is revealed that there is a Marine recruiting poster on the side of the truck. (The US Marines would never advertise on an army truck.) There is certainly a competitiveness in today's U.S. armed services; back in 1917 President Woodrow Wilson created the Committee On Public Information. This committee tasked "The Society of Illustrators" out of New York to create artwork that illustrated the demands that the Great War was placing on America and on American servicemen and women. These pieces of public art were created by some 300 of America's top designer's and were ordered to be placed on "every" wall and publicly viewable surface. Given the great need for the American public to support the war effort during The Great War, it is not unreasonable that an Army vehicle would show it's patriotism by sporting a U.S. Marine recruitment poster.