I was wrong, Anderson commanded only 85 men, of which 8 were musicians. Spherical case shot is a fused round, which can be set to explode at a predetermined range, showering the target with shrapnel. This round is used against artillery batteries and ships crews. Solid shot is just that, a sold iron ball. It is for punching holes in ships.
All of Andersons fuses had be captured when the Confederate forces occupied the Federal arsenal in Charleston. Without the fuses, case shot is the same as solid shot, it wont explode. Beauregard had 6000 men and 45 big guns to counter any threat Sumter offered. The fort was built to protect the entrance to the harbor. Most of its guns faced seaward, away from Charleston. At any one time, Anderson could only man 10 guns. Davis, Beauregard and Pickens, all recognized the fact that while an imposing structure, Sumter was of little real threat to Charleston. Hardly in a position to “besiege” Charleston. On the night of April the 12th, Anderson told Beauregard’s representatives, he would surrender the for at noon an April the 15th. Davis decided not to wait those two days.
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.
I forgot to reply to the above part of your post. Anderson knew that Lincoln's ships were supposed to arrive momentarily. Why should he surrender if he would have (he thought) lots of assistance from Lincoln's men and war ships? Here, incidentally, was what Anderson pinned to Washington when he learned the Union ships were headed his way [my black bold and red bold below]:
I had the honor to receive by yesterday's mail the letter of the honorable Secretary of War, dated April 4, and confess that what he there states surprises me very greatly, following as it does and contradicting so positively the assurance Mr. Crawford telegraphed he was authorized to make. I trust that this matter will be at once put in a correct light, as a movement made now, when the South has been erroneously informed that none such will be attempted, would produce most disastrous results throughout our country.
It is, of course, now too late for me to give any advice in reference to the proposed scheme of Captain Fox. I fear that its result cannot fail to be disastrous to all concerned. ...
... I ought to have been informed that this expedition was to come. Colonel Lamon's remark convinced me that the idea, merely hinted at to me by Captain Fox, would not be carried out. We shall strive to do our duty, though I frankly say that my heart is not in the war which I see is to be thus commenced. That God will still avert it, and cause us to resort to pacific measures to maintain our rights, is my ardent prayer.
The arm-chair generals at the New York Times couldn't understand why the South hadn't moved on Fort Sumter earlier, since they [the NYT] thought the arrival of Lincoln's ships in Charleston would defeat the South. They seem to ignore that the South wanted to reach a peaceful solution and had kept commissioners in Washington until the last minute trying to achieve that end. Here is what the New York Times said in their April 12, 1861 edition:
Sumpter [sic] on the one side and the Fleet off the North Channel on the other, will effectively cover any relieving expedition, whether of open boats, tugs, or small vessels, from any maritime attack, and confine all resisting operations to the land batteries. Experience has shown -- as in the case of Gen. WILKINSON’S passage down the St. Lawrence during the last war [the Mexican War doesn’t count as a war in the Times view?], with five hundred boats, suffering but a trifling loss, in the face of strong shore batteries – that batteries cannot effectually prevent the passage of an armament. Still less can be done when the batteries themselves will be exposed to such a terrific fire as Major ANDERSON can for some hours at least, pour with his whole force on Moultrie and the battery near Cummings' Point, the only two places from which boats or light draft vessels can be fired upon to any purpose.
But ANDERSON’S fire will not be the only one to which Moultrie may be exposed, as the smaller vessels can take with impunity positions from which shell may be thrown with great effect. No matter how brave or skillful the Southern troops may be, they will be under a fire which will render the entire stoppage of relief to Fort Sumpter [sic] nearly impossible.
... Why the Southern Commander, be he JEFFERSON DAVIS or Gen. BEAUREGARD, has delayed pouring on Sumpter [sic] his full force, and crushing it beneath an iron hail, if he could; why he has waited until, instead of concentrating his fire in security on one small point, he now has to defend a long straggling line [ten miles of shoreline], from a powerful fleet, it is impossible to tell. The reason may have been political; it may have been that there was not the vaunted readiness; it may have been incompetency; and it is not impossible that when the yawning abyss opened before them with all its horror, they may have lacked the insane courage required for the final leap.