On November 8, the Union commander before Anderson, Colonel John L. Gardner, tried to take all of the small arms ammunition from the Arsenal with troops dressed in civilian clothes so as not to alarm Charlestonians. The owner of the wharf they were using spotted them and threatened to raise an alarm. The ammunition was returned to the Arsenal.
Buchanan and his cabinet decided to replace Gardiner with Anderson and put a US Army Colonel or Major (my books list his rank both ways) Huber in charge of the Arsenal. In December after Anderson's arrival, Capt. Foster tried to get 100 muskets from the Arsenal ostensibly for workers in Fort Sumter and Castle Pinckney. Huber refused saying he would need orders from Washington to allow that. The Secretary of War deferred the order. A couple of weeks later, Capt. Foster used a previous order made by Gardiner to obtain 40 muskets. There was an uproar in the city, and the muskets were returned because Huber had said that no arms were to be removed from the Arsenal.
Huber's position was consistent with the agreement between South Carolinians and Buchanan to not change the strengths of the various forts in the harbor. The agreement was widely know by Charlestonians. Despite Buchanan's later public disavowal of such an agreement after Anderson moved to Sumter, two of Buchanan's cabinet members later revealed Buchanan's reluctance in cabinet meetings to go back on his promise to the South Carolinians.
FYI, after the surrender of Fort Sumter to the Confederates, Huber resigned his position in the US Army and joined the Confederacy, later becoming a Major General.
You are correct that Fort Sumter's canons did not face Charleston, but like Anderson said, he could command the harbor with his guns. The harbor was essential to Charleston and South Carolina, and they did not want their future commerce and tariff revenue under the control of a possible hostile fort.
Fort Sumter was not initially in good enough condition to repulse an amphibious attack from the city. There were low openings in the walls of the fort, and, as you said, the canons faced seaward. Anderson was concerned about those weaknesses, and I don't think he could have held the fort if South Carolina had wanted to take it at that point. The state did not move against the fort then, but demanded that President Buchanan honor his promise to Southern Congressmen not to change the relative strengths of the forts in the harbor.
Huger was dumped by Lee after the 7 days battles and sent out to a staff position in the Trans-Mississippi Department.
Fort Sumter was of marginal military threat to Charleston.
What Fort Sumter really was, it was a symbol of continued Federal presence in what was now the Confederate States of America. That galled the South Carolinians and it galled the new President of the Confederacy. Had the North tried to force a resupply of the Fort, they would have been see as the aggressors, even if the resupply was only provisions. Davis screwed the pooch. He decided to reduce the fort by force, rather than allowing Anderson to surrender in a couple of days. His forces were probably adequate thwart any attempt to resupply Sumter. If he had waited, allowed the North to try a forced resupply, He would have been in the right, to fire on that force and the fort. He chose instead to start a war that did not need to start at that point. His firing on Fort Sumter gave Abe Lincoln all the ammunition needed to call for crushing the “rebellion”. JMO