"Lowcountry" by Abby Rosebrock, author of 'Blue Ridge" seen at Atlantic Theater Company in 2019, has a great deal going for it: a fine cast, a play told in real time, scenic design in keeping with the milieu and the plot, and characters quirky enough to keep us interested. However, this talky play doesn't get where it is going until the last ten minutes and has a great many unanswered questions that perplex as one wa…