Answer: Alfred and Klara were reunited
The film "The Shop Around the Corner" had a happy ending. After everyone left, Alfred and Klara were alone in the shop together. Both of them told each other about their plans to go out with their respective dates, and Klara told Alfred that she began to like Alfred when they met each other for the first time, and Alfred told her that he met her "mystery correspondent" and that he found out that he was bald, overweight and unemployed. However, it was just an act he put up, and finally, he revealed to Klara that he himself was her "mystery correspondent" and her "dear friend" and her pen pal. They kissed each other, and the film ended. Thus the film had a happy ending.
As regards Barbara Stanwyck, she and Jimmy Stewart never worked together in any films or TV shows, even though they were almost contemporaries of each other. Even in "The Shop Around The Corner", Barbara Stanwyck didn't play any role in the movie, and Jimmy Stewart's character in the film, Alfred Kralik, didn't have any childhood friend whom he married at the end of the film.
As regards Cary Grant and Katharine Hepburn, they worked with Jimmy Stewart, but neither of them acted with Margaret Sullavan. The film in which Cary Grant, Jimmy Stewart and Katharine Hepburn appeared together was "The Philadelphia Story", released in the same year as "The Shop Around the Corner". However, Cary Grant and Katharine Hepburn didn't make any guest appearance or special appearance in "The Shop Around the Corner", nor did they appear in the film as the second lead pair, or in supporting roles since the film was a solo starrer project.