As we know that South Carolina did not seceed until December 20th. Well after it was known that Lincoln had been elected. And well after the time needed for Scott to have his conspirators in place to deal with South Carolinas secession.
Once South Carolina did seceed that plan would have to be executed well before Lincoln was sworen in on March 4th. They could not give South Carolina a grace period to seceed and form a new country before they acted. Because others, and they did not know how many would, may follow South Carolina example.
The funny thing that their plan, to cut South Carolina off at the pass, so to speak, and stop this fever before it started by demonstrating the power of the United States government, backfired. The reaction of others states was something, I believe, that Lincoln didn't completely foresee and complicated the realitively simple plans of threatening to blockade Charleston Harbor and there by forcing South Carolina, who would be standing alone, to rejoin the Union as they did in 1832. The fact that they almost totally circumvented President Buchannan and Secertary Floyd indicates the power of "The Office of The President Elect." Especially with the inside help of Winfield Scott.
"The Star of the West" incident was seen by the other southern states for exactly what it was, a threat to the soverignty of the State of South Carolina, and the same could be done to them.