The Oscar Wilde quote comes from his 1895 play The Importance of Being Earnest and reads: "To lose one parent, Mr Worthing, may be regarded as a misfortune; to lose both looks like carelessness."
In the original story "The adventure of the Noble Bachelor", Inspector Lestrade was investigating the case. Since Colin Jeavons, who played Lestrade in all of Granada's Sherlock Holmes productions, wasn't available, another police detective took his place with Lestrade's absence being explained as him being at a spa.
Anna Calder-Marshall (Lady Helena / Agnes Northcote) is the wife of David Burke, who played Watson in The Adventures of Sherlock Holmes (1984).
There is a misconception that there are two different actresses, both named Mary Ellis, with a great age gap between them. There is not. There is one Mary Ellis. The confusion arises in the character's names. Mary Ellis plays Lady Florence, the Lady toasting wine with Hetty's father's bourbon. Flora is a different character. Flora may be used as a nickname for Florence but it's not the case here.