The word "if" doesn't give you a choice?
The "Agent" had informed Beauregard that a Union fleet was sailing at them with orders to attack them.
Beauregard could have abstained from firing on the Fort if he didn't believe the man. The problem was, the orders sent by the navy for all those men and ships said exactly that.
No rational man would have believed that all those ships given that order would refrain from obeying it.
Clearly both Davis & Beauregard believed what they believed, regardless of Lincoln's words.
So there was no actual choice.
DiogenesLamp: "The problem was, the orders sent by the navy for all those men and ships said exactly that."
But Lincoln's final orders, as we have reviewed before, said exactly what he told SC Governor Pickens -- no use of force or reinforcement if no Confederate resistance.
So clearly Jefferson Davis' use of "if you have no doubt" roughly corresponds to RE Lee's use of "if practicable" in his Gettysburg orders to Second Corps commander Ewell about Culp's Hill.
Ewell decided it was not "practicable" and so lost the whole battle for Lee, while Beauregard decided he had no doubt and won the battle which started the war that destroyed the Confederacy.
Such "ifs" were just the way Southern gentlemen addressed each other in those days.