Bajie was still in the middle of his sighing when he saw a flash of firelight behind the canopy.

一幌 is a single flicker. The interruption arrives at precisely the last syllable of the lament, which is how this novel usually signals that a scene has finished its work and can be cleared away.

The idiot said: "There must be someone behind there tending the incense fire."

侍奉香火之人 picks up 侍奉 from the third line of the lament he has just this moment finished — nobody comes to wait on the dead man, so there must be somebody here waiting on him. Bajie is still supplying inhabitants for an empty house.

He stepped across quickly, went round the canopy and looked — and it was light coming through the lattice of a window that pierced the storey.

穿樓的窗扇 has 扇 as shàn, the leaf or panel of a window. The firelight gets explained away, and the explanation is perfectly correct. The danger is somewhere else entirely.

On that side stood a table in coloured lacquer, and on the table lay a few brocaded padded garments in a heap.

亂搭著 uses zhe for a continuing state — draped anyhow, left lying. 彩漆 is coloured lacquer. Clothes left out on a table, in a house with nobody living in it.

The idiot picked them up to look, and they were three brocade-quilted waistcoats.

納錦 is quilted brocade; 背心兒 is a sleeveless waistcoat with the little er ending. Three of them, for the three people who will shortly be standing there to put them on. The count is the trap.