I know all that about Hastings, Ariadne, Miss Lemon, Race and Battle, It is just my opinion, you Like Endless Night whereas I found it dry and lifeless and in need of a Recurring Character to give it a lift and sparkle,
I would not put Miss Marple in Halloween Party, If I was to do anything with it maybe I would swap Poirot for Luke from Murder is Easy or Calgary from Ordeal by Innosense I definitely would not take out Spence or Elspeth who are in my opinion the best Characters in the Book apart From Poirot and Ariadne
I would like the Beresfords in "Why Didn't They Ask Evans" - I feel sure that Tuppence would like Bassington-ffrench as well.
There are not too many steps from Miss Lemon to Miss Marple, Miss Lemon to Ariadne to Dane Calthorp to Miss Marple so it is conceivable that Miss Hubbard would know Miss Marple, Good Idea HerculeAndAchille
She would Phone Dermot just like she phones Dolly in a Book where Dolly is only mentioned.