May one comfort a mourner after Shiva if one did not do so beforehand?
Comforting an Avel for the passing of a parent: One who is in mourning for a parent may be comforted by others for the first 12 months. After 12 months, he is not to directly comforted for the loss. This means that the person comforting him is not to mention the name of the deceased to the mourner, but is to simply say “Tisnachem/Be comforted.” [The custom today, however, is not to say even “Tisnachem.”]
Comforting an Avel for other relatives: One who is in mourning for other relatives may be comforted by others for the first 30 days. After 30 days he is not to directly comfort him for the loss. This means that he is not to mention the name of the deceased to the mourner, but is to simply say “be comforted.” [The custom today however is not to say even “Tisnachem” after the Shloshim.”]
Comforting an Avel for the passing of his wife: One who is in mourning for his wife, and has since remarried, may not be comforted for her passing inside his home. Outside his home, if he is met outside, one may say “Be comforted” in a soft tone and with a heavy heart. If he did not remarry, he may be comforted [regularly] until three festivals pass from the time of burial. [After the passing of the three festivals, he is not to be comforted at all for the loss, as his pain has already dissipated.]
 Michaber Y.D. 385:2; Tur 385; Rosh Moed Katan 3:38; Moed Katan 21b; Miseches Semachos 14; Omitted from Rif and Rambam; See Gesher Hachaim 1:212; Pnei Baruch 11:13; Nitei Gavriel Vol. 1 86:4; Vol. 2 11:1-3; See Hiskashrus him 468:14; 475:14
Is the Mitzvah of Niuchum Aveilim fulfilled after the Shiva? See Nitei Gavriel Vol. 2 11 footnote 1 that it is unclear if the intent of this Halacha is to say that one fulfills the Mitzvah of Nichum Aveilim even after Shloshim or 12 months, and if the comforting is an obligation or not.
The Nussach to say after Shiva-The Rebbes directive: In a letter written to a widow after the loss of her husband, having left her with four young orphans, the Rebbe mentions that the common Nussach of “Hamakom Yinacheim…Besoch Shaar Avlei Tziyon Veyerushalayim” is to only be recited during the actual Shiva and not afterwards, being that afterwards begins the period of comfort. [Igros Kodesh Vol. 31 p. 310] Vetzaruch Iyun as to how this fits in with the above law which permits comforting throughout the first year or at least for the first 30 days for other relatives. Indeed, the Rebbe Rayatz himself wrote a letter of comfort to the Rebbe after the passing of his father, and included within it the common Nussach of Nichum Aveilim “Hamakom Yinacheim…Besoch Shaar Avlei Tziyon Veyerushalayim” even though the letter was written after the Shiva. [See Menachem Tziyon 1:39]
 Aruch Hashulchan 385:3; However, see Nitei Gavriel ibid footnote 3 that seemingly the Aruch Hashulchan’s ruling refers only to other relatives after Shloshim, while an Avel for a parent is comforted with Tisnachem after 12 months.
 Aruch Hashulchan 385:3
 Aruch Hashulchan 385:3