I believe 'A few paces off in' is functioning as a propositional phrase in this sentence .... it can be put next to darkness because darkness is a noun. A 'few paces off' is in reference to the speaker, who is not included in the sentence, so you could think of it like this: (From where I stood) A few steps away in the darkness was a shadowy figure.... Another example of this sort of usage might be something like: Several meters into the sea there was a shark waiting for hapless swimmers.

Paces are steps....the short definition is "close" or "very near." "off" means "away" "against" means "touching", it has more information than "by"; so no, because you lose information.

one = single, two = couple, three (or about three) = a few Paces = steps. "Off" in this sentence, means "away" A few paces off = A few paces away "Against" the wall suggests that a person might be actually leaning against or touching a wall. "Leaning----Against----A----Wall, (or "the wall) would be a common construction. However, "by" the wall could have been used just as well. By the wall and against the wall are more or less, equivalent (the same). To show proximity (nearness) several words can be used almost interchangeably. (A) Against (B) By (C) Near (D) At