No idea what he should have done. I wasn't there. But the commander of Fort Moultrie moved, without orders, to the unfinished Fort Sumter in December. There was no attack and they were free to leave at any time. In January, Lincoln attempted to send supply ships to FS. Shots were fired across the bow to turn them around. Anyone with a semi-functional brain would understand that the plan was to wait them out. There was no food supply on FS. But still no attack on troops. In April, Lincoln ordered a fleet of warships, with reinforcements, to FS. And the Civil War started. Seems "defending" an unfinished fort was rather stupid. As late as early April, the troops could have peacefully withdrawn. Maybe the Civil War happens anyway. I'm sure it does. But, as best I can tell, Lincoln became obsessed with taking/keeping Fort Sumter. THAT seems to be a stupid reason to kill thousands of Americans.
I think telling the enemy the time frame for withdrawal is stupid. If you're going out of town, you don't leave a note on the door for the burglars. He is pandering. I think Gitmo was going to be closed by year end. That's not happening either but it's what people needed to hear.