"and she shall sit in your house": the house that he frequents, so that he "stumbles" upon her coming and going, and sees her in her ungainliness, and she becomes repulsive to him.
"and she shall mourn her father and her mother": This is to be taken literally. These are the words of R. Eliezer. R. Akiva says: "her father and her mother" are idolatry, as it is written (Jeremiah 2:27) "They say to the wood 'You are my father,' and to the stone 'You bore me.'"
"a month of days": thirty days. R. Shimon b. Elazar says: "a month": one (month). "days": two (months). "and after that": three (months, all together), for the "amendment" of the child, to distinguish between the seed of the first and the seed of the second (i.e., It is possible that she was already pregnant by some other man, and we do not know whether this is a nine-month child of the first or a seven-month child of the second. But if he waits three months, this doubt does not exist.)