When the chazan reaches Modim, the whole congregation bows with him and recites Modim d’Rabbanan, whose nusach differs from that of the Modim in the Amidah, as clarified in the Talmud (Sotah 40a).
The entire congregation bows down in Modim d’Rabbanan. The laws concerning this bow are similar to the laws of Modim in the silent Amidah (Mishnah Berurah 127:2; Kaf HaChaim 1; see earlier in this book 17:6).
There are those who say that it is necessary to bow again at the conclusion of Modim d’Rabbanan. Others say that it is proper to recite the full Modim d’Rabbanan while bowing. The prevalent minhag is to bow only in the beginning of Modim d’Rabbanan, as was the minhag of the Ari (see Shulchan Aruch and Rama 127:1; Kaf HaChaim 10).
In a prayer service in which Birkat Kohanim is recited, as in Shacharit, Musaf, and in the Minchah of fast days, if there are no Kohanim present, the chazan recites the verses of Birkat Kohanim as a prayer, and the congregation responds “Ken yehi ratzon” to every verse.
There are two versions of Birkat HaShalom – “Sim Shalom” and “Shalom Rav.” According to Nusach Sephard, which follows the Ari, Sim Shalom is recited in all the prayers. According to Nusach Ashkenaz, Sim Shalom is recited in a prayer in which Birkat Kohanim can be recited. In a prayer in which Birkat Kohanim cannot be recited, Shalom Rav is recited. If one mistakenly said the wrong wording, he still fulfilled his obligation (Rama 127:2, Mishnah Berurah 13; Kaf HaChaim 24).
If a chazan becomes disoriented to the point where he cannot continue praying, the congregation waits to see if he can return to his senses. If he cannot continue, another chazan is appointed to replace him. If this happens in one of the middle berachot, the second chazan starts from the beginning of that berachah. If it happens in the middle of the first three or the last three berachot, he starts from the beginning of those three berachot (Shulchan Aruch 126:1-2).
. Bei’ur Halachah 127:2, s.v. “Aval” writes that if he remembers in the middle of the berachah that he began Shalom Rav in Shacharit, he must go back and correct it as long as he did not finish the berachah, because the wording of Shalom Rav is short and lacks some of the content mentioned in Sim Shalom. However, if in Ma’ariv someone realizes in the middle of the berachah that he mistakenly started Sim Shalom instead of Shalom Rav, he need not go back and correct it because Sim Shalom includes the wording of Shalom Rav. The Nusach of most Chassidim is to recite Sim Shalom every day in Minchah. It seems that the reason for this is because on fast days Birkat Kohanim is recited in Minchah, since there is no concern of intoxication then. If so, we see that essentially, it is befitting to recite Birkat Kohanim in every Minchah when there is no concern of intoxication; therefore, it is appropriate to recite Sim Shalom.