Hour One: the sitters' book
Nudge. This is twenty years after the sitting. What did Kit's letter tell his father to say, if anybody told him Kit was in the house?
Harder
In 1922 you read Kit's last line from his signaller's card, dot dash dash dot first, and told Edmund what it said.
The answer
The word is peace.
In Part 2 you decoded Kit's line in Morse, .--. . .- -.-. ., and told his father: peace. Doyle wrote it in the sitters' book, and Edmund blotted it out.