Funny, that is just what I was thinking about you! You know you sit here and quibble about details of which Forts got abandoned, but it occurs to me that the British abandoned quite a lot of Forts. This is a normal part of what happens when one part of a nation separates from another.
Why should one country maintain forts inside of another against that nation's consent? Did we not abandon our Military bases in Iran? How about the Philippines? That was once ours, but we let it go and we sought and got leases to remain, but when they wanted us out, we left.
Same thing with Cuba. Pity we couldn't show the same courtesy to people who were our wrong headed kinsmen.
The important point to remember about Union troops in the Union Fort Sumter is that Brits also maintained their troops in their forts in United States Northwest Territory for over 30 years (!), from 1781 through 1814.
British troops in British forts in United States territory were doubtless matters of some concern to our Founders, but the Founders never made them a casus belli, never threatened war if forts weren't abandoned, and never fired on those forts to force their surrender.
Seems our Founders understood something the Confederates didn't.