"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 ... "
There is no reason to expect that a single noun phrase could substitute for "it" in all cases. "It" refers back to a coreferential antecedent, which will be described differently by different speakers. What matters is that the thing referred to is the same, not that it is described in the same way.
1) "Where's my book?" "It's on the table." it = your book
2) "Isn't my kitty precious?" "I think it's evil, but whatever." it = your kitty
3) "This Rubik's Cube is driving me crazy!" sigh "Give it to me." it = that Rubik's Cube
Best Answer
"would" is subjunctive, so it's a possible future. All of the phrase should match the subjunctive state of they are all possible, but not yet reality.
If she did the earning and enjoying, then only part of the statement is subjective: the "would" implies a contrast. She did earn, but we expected her to work diligently [but she did not].
So it depends what one intends