(81)break down
Emily Brent said:
“I think that the accusation was true. You all saw her last night.
She broke down completely and fainted.

「break down」には「(機械などが)故障する」「失敗する」「(人が)取り乱す」「(健康などが)衰える」等の意があります。ここでは文脈から「(人が)取り乱す」の意。

(82)bring home to …
The shock of having her wickedness brought home to her was too much for her.
「bring home to …」は「・・・にしみじみ悟らせる」の意。

(83)if you prefer
Emily Brent said quietly:
“Call it if you prefer, an Act of God.”

「if you prefer」は「その(次のことの)方がよければ」の意。

(84)polish off
Rogers and his Missus polished off that old lady.

「polish off」はここでは「殺す」。その他「をさっさと終えてしまう」「を負かす」等の意になることもあります。

(85)That’s as may be.
「That’s as may be.」は「そうかも知れない」。