Learn English – “Faster, not sooner”

In theory, this should be easy. Drop "deceptively" and you have the essential quality of the subject:

"The floor is deceptively flat" → The floor is flat.
"He's deceptively strong" → He is strong.
"He looks deceptively strong" → He looks strong.

Now add "deceptively" back in to indicate how the observer has been deceived:

"The floor is deceptively flat" → The floor is flat (but appears otherwise).
"He's deceptively strong" → He is strong (but appears otherwise).
"He looks deceptively strong" → He looks strong (but is otherwise).

The difference between #2 and #3 is the difference between "looks" and "is." This has the effect of flipping the comparison around: in #2, you are deceived about how he is, and in #3 you are deceived about how he looks.

In theory, it's easy. In practice, you're just as likely to find the word used to exactly the opposite effect, so in most cases you should assume you're going to need to get the meaning from context. I'd recommend staying away from "deceptively" entirely when writing: "The floor is flatter than it looks."

"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 ... "