Could Agatha Christie have been the mystery writer who completed Annie Haynes' The Crystal Beads Murder, left unfinished when Haynes died at the age of 63 in 1929?
By 1929, however, she was coming out of this bad patch. She published The Seven Dials Mystery, which I personally think holds up well today, and had written some Tommy and Tuppence and Miss Marple short stories, rather delightfully jolly and keen affairs respectively. Would she have found time to complete Annie Haynes' unfinished mystery? And, if so, would she have had any inclination to do so?
Annie Haynes's publisher, The Bodley Head, published Christie's first five novels, between 1920 and 1925. Yet Christie left The Bodley Head in 1926, after considerable acrimony, jumping ship with The Murder of Roger of Ackroyd to Collins and sticking it out with them for the next half-century. I have a hard time believing that Christie in 1929-30 would have wanted to have had anything to do with The Bodley Head.
Also, had Christie completed The Crystal Beads Murder, it's hard for me to believe that we wouldn't know this by now, with all the people who have intensively researched her life and works, like John Curran.
Conclusion: On the whole Christie seems unlikely.