Page 176. " The Lord is mindful of his own "
German postage stamp honoring Felix Mendelssohn
Public DomainGerman postage stamp honoring Felix Mendelssohn

"The Lord is mindful of his own" is not a direct quotation from Scripture. Emily Brent may be thinking of Mendelssohn's oratorio St. Paul, which premiered in 1836; one of the ariosos has this title. (You may hear a children's choir singing it here.) Or she may be thinking of the verse on which the arioso is based: Psalm 115:12:

The LORD hath been mindful of us: he will bless us; he will bless the house of Israel; he will bless the house of Aaron.

"Thou shalt not be afraid for the terror by night" is a quotation from Psalm 91:5.

Page 184. " But that bee? It can't be coincidence? "
Bumble bee

This is a reference to the fifth stanza of the rhyme:

 

Six little Indian boys playing with a hive;
A bumblebee stung one and then there were five.

Page 190. " a sixth broken Indian boy "

This is apparently a mistake. The text should read, "a fifth broken Indian boy."

We have had five deaths (Marston, Mrs. Rogers, Macarthur, Rogers, Brent) and five smashed figurines so far, not six.

Page 191. " They were reverted to more bestial types. "
Creative Commons Attribution"A big bear hugged one ..." - Credit: animalphotos.info

Christie ratchets up the animal imagery here, preparing the way for the eighth stanza of the poem:

 

 

Three little Indian boys walking in the Zoo;
A big bear hugged one and then there were two.

Page 194. " The hearse is at the door, sir. "
Horse-drawn funeral hearse with driver, circa 1900 courtesy of Neil Regan Funeral Home, Scranton, PA
Creative Commons Attribution Share AlikeHorse-drawn funeral hearse with driver, circa 1900 courtesy of Neil Regan Funeral Home, Scranton, PA - Credit: Neil Regan Funeral Home

This is a quotation from "The Murderer's Haunted Couch," a poem by Thomas Lovell Beddoes (1803-49):

The hearse is at the door,
The widest gate of Hell is open for thee,
And mighty goblins summon thee to Death.—
Come down with me!

Page 195. " The engine's not been run to-day since Rogers hasn't been there to see to it. "
Early 1900s power generator
Public DomainEarly 1900s power generator

This is a reference to a generator that would have been run to provide the household with electricity.

Page 196. " The smell of the sea at St. Tredennick "

There are two Tredennicks, both in Cornwall (Tredennick, St. Issey, Wadebridge; Tredennick, Liskeard), but neither is on the sea and neither is referred to as "St." Tredennick.

Nor does the Catholic Church recognize a "St. Tredennick".

I am assuming the location is a Christie invention.