Last August, when the pandemic seemed endless, John R. MacArthur, the publisher of Harper’s, asked his doctor about the virus. “I haven’t seen a case in three months,” his doctor told him. Mr. MacArthur ordered up some office air filters, put arrows on the floor and brought his small staff back to work. “We’re better off working together,” he told me in an interview inside Harper’s headquarters, an underpopulated warren on Broadway north of Houston Street, in mid-March. “And I’m happier here.” What has ensued at Harper’s since last summer is a kind of hostage situation, in which the magazine’s 17 newsroom staff members reluctantly trudge into work, try to keep office windows open and Zoom one another from their desks.