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