"coiner" - one who counterfeits coins. Not something we hear of much today, but in earlier times was a reasonably familiar sort of criminal.
"box-room" - a room in a house (usually a small room) which is just used for storing things, usually things that are not wanted very often. The collocation "box-room attic" is unusual, because these two words mean nearly the same thing. (Not quite, because a box-room could be anywhere in a house, not just under the roof).
"cistern": yes, it is still quite common for British houses to have a water-tank in the roof space - sometimes two (hot and cold). This was partly to provide a head of pressure for taps and showers, and partly to guard against interruptions in supply (and in the case of hot tanks, to provide a reservoir of heated water for when you needed it quickly, as in a bath).
There is not a literal tunnel, it is saying that between the tank and the sloping roof there is a long and narrow dark place (dark because it is behind the cistern).
"of course" is a parenthetical remark meaning "as you already know", or "as is obvious". It is much more common in speech than in writing, but of course this writing is meant to suggest somebody is speaking, and telling a story.
"For" says that the sentence is a reason or explanation of what precedes. So the meaning of "For of course he was thinking .. " is something like "(He was excited) because, as you probably realise, he was thinking ... "
In the excerpt, editorial is essentially being used as a synonym of commentary. It's odd that neither of the (generally good) dictionaries you looked in mention this metaphorical meaning. Dictionary.com lists it as the third definition (emphasis mine):
- an article in a newspaper or other periodical presenting the opinion of the publisher, editor, or editors.
- a statement broadcast on radio or television that presents the opinion of the owner, manager, or the like, of the station or channel.
- something regarded as resembling such an article or statement, as a lengthy, dogmatic utterance.
This usage of editorial usually has mildly uncomplimentary connotations, either because the statement of opinion was too lengthy, or because (as in the novel) it was being expressed in an unwelcome setting.
Best Answer
Siberia was the location for several prison camps (gulags) in the USSR. It is largely uninhabited and desolate. According to Wikipedia, the population density of Siberia is about 8 people per square mile. It is heavily forested and most of the people who make Siberia their home are disconnected from the rest of the world.
The lyricist is painting this picture of the heart going into isolation and doing so in a manner much more extreme than by sulking in a corner. It's not necessary that the heart went to prison, only that it went to a vast, desolate place where it would be nigh impossible for anyone to find until it wanted to be found.