"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 ... "
The reason you can't use "twice bigger than" is simply because this is a phrase we do not use. I am not sure exactly why, maybe someone else has a better reason than that, but I do not. The information presented by your grammar book is just about the grammatical correctness of each substitution, as opposed to making a sentence with a similar unambiguous meaning.
For the other pages you have linked, the question isn't of grammar, but in exactness and level of ambiguity.
Referring to your last question about "two times loner than ... " vs. "two times as long ..." This will confuse native speakers too. They are saying that the difference in the usage is that in the first, "two times" describes how much longer and in the second, "two times" describes the actual length.
I tend to disagree, and think that the first usage is ambiguous enough for most people that it shouldn't really be used.
As other posters in the answers you have linked point out (and is reinforced by the difference in answers), using "times more than" form is pretty ambiguous, or difficult to ensure your meaning is accurately understood.
if X and Y are numbers
"Y is two times as big as X" --> Y is 2*X (Y: 4, X: 2)
"Y is 200% of X" --> Y is 2*X (Y: 4, X: 2)
"X has increased by 200% to become Y" --> Y is X+2*X (Y: 6, X: 2)
"Y is two times bigger than X" could be referring to both of the situations above (Y being 4 or Y being 6.) Some may argue one way or the other, but the fact that it is unclear is definitely apparent.
Best Answer
Your understanding is correct...this use is called "subjunctive".