this is fun - I’m pleased. it’s from 1928, it’s a Locked Room Mystery, and the style plus characters plus plot have got me thinking of Miss Silver books, with a bit of Carr’s The Crooked Hinge (though I hope this is better). the author had 12 Mysteries published 1923-1930 - that includes posthumously, because she died relatively young in early 1929 (Detection Club formed one year later) - and they disappeared into limbo not long after initial publication. thanks to the Intro (Intros, really) for this info, and the other bits of data dug up about Annie Haynes.
and yes - Miss Lavinia is a pip of a character, and could have done well in her own series. I wait on every drop of her verbal venom.