How important was the role of Lenin, a leader in the October Revolution, in the Bolsheviks' seizure of power in 1917?
Lenin's importance to the Bolsheviks' seizure of power was in his ability to persuade and lead. He was not there at the start, and he was not the military leader, but he was important in getting both the masses and the leaders to act decisively.
When Lenin was brought out of exile in Switzerland to Russia by the German government to undermine the Russian effort in its war against Germany, he gave many speeches pushing the people to rise up against the Provisional Russian government. These speeches did not start the revolution, but they encouraged it and got more converts. After he and other leaders had to flee, he returned and played a big role in persuading other leaders of the Bolshevik movement that the time was now right and they would be able to win.
So, even though Lenin was not in Russia for the start of the rebellions and even though he did not lead the military forces, he was important to the Bolsheviks.