He might have to do another flashback. Like the tale of Cuthbert dying (unless that's talked about later in the last book).
The Battle of Jericho Hill. He never did tell that story, so yeah, good call. Going into more detail about The Good Man John Farson.
I haven't read any companion novels. Do you mean the novels King references? The Stand and Salem's lot most notably.
Salem's Lot just has Father Callahan. The Stand is notable for the scene in Wizards and Glass and because Flagg is the main antagonist. No, the companion novels are:
Black House - where Lord Munshun kidnaps kids who are breakers, and is the continuation of the story of Jack Sawyer, who we met in The Talisman. An excellent book.
Low Men in Yellow Coats (found in the collection Hearts in Atlantis) - the story of Ted Brautigan after he escapes from the cantoi, or Low Men.
Insomnia - the story of Ralph Roberts who must save Patrick Danville. THe Crimson King shows up briefly.
Little Sisters of Eluria - a short story of Roland early in his quest.
IT - a few times the Turtle is mentioned and there are a few hints of the tower. Some believe It to be Dandelo (if you don't know who this is, don't worry)
oh fuck it, just go here:
http://www.stephenking.com/darktower/connections.html
the ones you MUST read are Black House, Insomnia, and Low Men in Yellow Coats