Certainly Beauregard wanted Sumter in the best condition possible, but I do not believe that Beauregard held back because of that reasoning.

Beauregard was well aware of Anderson's tactical situation, his food supply, numbers of men, armaments and muntions. He knew that he was well able to reduce Sumter at any time he wished so long as Anderson did not recieve the necessary men and supplies to properly man the forts defences.

After all it took the Union navy several years before they could reduce the fort. Not because it was so strong a position, but because it was a part of an interlocking system of defences, which would draw a crossfire from the other fortification in the harbor when anyone attacked it by sea.

As far as "saving face" Anderson had already informed Beauregard and Lincoln that he would have to surrender the Fort by the 15th of April due to lack of food. This is strange becauses in March he had reported that he had enough salt meat for 48 days rations. He did not clarify if that time frame was at full rations or half rations. He was prepared to surrender the fort and had negociated that with Beauregard until the arrival of the "relief effort" outside Charleston Harbor.

Beauregard was simply honoring the agreement that Anderson was preparing to surrender the fort if he was not resupplied. remembering also that Beauregard was constrained because of the various peace negociations that had been going on up until early April between the Confederates States government and Lincoln. Davis had in fact told Beauregard that if the fort was goin to be surrendered by that time NOT to attack it, unless the Fort fired on the Confederates or was going to be resupplied.

Beauregard was not absolutely sure how large this relief effort was going to be. And he wasn't sure if he could stop them with his batteries before the supply ships could reach Fort Sumter. The only solution to his orders and situation was to fire on Fort Sumter.