^ 5.The minhag of the Ashkenazim is that the congregation starts their Amidah after Birkat HaKel HaKadosh, and according to the Sephardic custom, the congregation starts along with the chazan. The basis for this difference is in how each custom relates to the berachah of Hakel HaKadosh. The poskim disagree as to whether the halachah follows the Yerushalmi, which maintains that there is special virtue in answering Amen to the berachot HaKel HaKadosh and Shome’a Tefillah. According to the Rama, a person may respond to their recital even if he is in the middle of saying Birkot Keriat Shema, whereas according to the Shulchan Aruch 66:3 the law regarding them is like the laws concerning all other berachot to which we may not respond while in the middle of reciting Birkot Keriat Shema. Therefore, according to the Mishnah Berurah, one starts his Amidah after Birkat HaKel HaKadosh so that he may respond Amen, and according to the Sephardim, there is no reason for this. This is clarified in Shulchan Aruch 109:1.
Even so, I have presented one approach for both Sephardim and Ashkenazim since there are numerous reasons why even Ashkenazim should start with the chazan in Shacharit: 1) L'chatchilah, it is best to abide by the ruling of the Shulchan Aruch, which rules that one does not answer Amen to HaKel HaKadosh in the middle of Birkot Keriat Shema, and if he starts reciting the Amidah with the chazan, he avoids this uncertainty. 2) From the standpoint of prayer in a minyan, there is virtue in the fact that all ten start praying together as explained in this book, chapter 2, note 2. 3) It is less complicated halachically to respond to Kedushah after Birkat Mechayei Meitim than in the middle of Birkat Emet V’Yatziv. 4) It is possible that reciting the Amidah in a minyan together with the chazan is considered as responding Amen (see Mishnah Berurah 109:13-14). In Minchah, it is easier to recommend to Ashkenazim who normally extend their prayer that they should start with the chazan enabling them to respond to the Kaddish after the Amidah, for certainly its virtue is no less than the virtue of responding Amen to HaKel HaKadosh. However, for those who shorten their prayer, even according to the Sephardim it is good that they start after HaKel HaKadosh, so that they will merit answering Amen after the berachot of Chazarat HaShatz, including HaKel HaKadosh, which some say possesses special virtue. Another reason for this is so that they will not be idle upon finishing the Amidah.

^ 6.The Mishnah Berurah 125:1 mentions both opinions and in section 66:17 he writes that in practice one may not respond the verse Yimloch while in the middle of Birkot Keriat Shema, for that is the opinion of most Acharonim, and so rules the Yechaveh Da’at 6:3. That is also the law concerning reciting this verse in the middle of the prayers of supplication after the Amidah (Elokai Netzor), as explained in Mishnah Berurah 122:4. However, Aruch HaShulchan 66:6 writes that this matter is left unresolved and one may do as he pleases. After bringing many Acharonim who maintain that Yimloch is not part of the main section of Kedushah, Kaf HaChaim 66:18 (and section 122:1 and 124:17), writes that Yimloch is, indeed, considered part of the Kedushah, as can be inferred from Sha’ar HaKavanot. This issue also has halachic significance pertaining to one who is in the middle of reciting the Shemoneh Esrei, whether he must also be quiet during Yimloch. See Aruch HaShulchan 104:13.

^ 7.The Bei’ur Halachah 125 s.v. "Ela" writes that if the chazan starts the verse before the congregation finishes saying it, he is considered to be reciting it in a minyan. However, if even then they will not hear him, the Bei’ur Halachah is uncertain as to whether the chazan may recite the verses after the congregation has finished, for perhaps because he is saying them in order to fulfill the obligation of the people listening, he need not say them with ten people, and it is enough that ten people hear him to be considered reciting them in a minyan. He leaves this matter unresolved. Divrei Yosef 13 writes that the chazan must say the verses with the congregation. However, Beit Yehudah, part 2, 3, writes that we are not strict that the chazan recite the verses with the congregation and that is also implied from Igrot Moshe, Orach Chaim 3:4.
When a person is praying the Amidah and silently stops so as to hear the chazan recite the Kedushah, as explained in Shulchan Aruch 104:7, yet does not succeed in hearing him, the Levushei Mordechai, part 1:17 writes that he should have kavanah to hear a congregant instead. However, many poskim maintain that since the people in the congregation do not have kavanah to fulfill him in his obligation, this is not effective. Therefore, if he cannot hear the chazan, it is best that he continues praying the Amidah. So is written in Kaf HaChaim 104:36 and implied in the Igrot Moshe there.

^ 8.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.

^ 9.The Shulchan Aruch 126:3 writes that a chazan who forgot to recite Ya’aleh V’Yavo in the Amidah repetition on Rosh Chodesh or Chol HaMo’ed does not repeat it. Although an individual must repeat his prayer following such a mistake, nevertheless, a chazan does not, since subsequently in Musaf, the specialness of the day is mentioned and therefore, so as not to trouble the congregation, we do not compel him to repeat Chazarat HaShatz. However, if he has not yet finished praying, he returns to Retzeh in order to insert Ya’aleh V’Yavo, for that is not such a big bother to the congregation. A chazan who errs in his silent Amidah does not need to repeat his prayer, since he fulfills his obligation in the Amidah repetition (Shulchan Aruch 126:4).

^ 10.Some say that when this individual, to whom the congregation agrees to respond, finishes Tachanun, he says Half-Kaddish, and when he concludes U’va L’Tzion, he says Kaddish-Titkabal. So write Kaf HaChaim 56:37 and Ishei Yisrael 34:4. The Hagahot Ish Matzliach on section 69 writes that some say that Kaddish-Titkabal is recited only when at least three people did not pray, and others say that it is only recited when six people who did not pray are present. Each community follows its own customs. As it seems, the prevalent minhag is to recite the Kaddishim after the Amidah only for six people who did not pray.
In principle, if the person who did not yet pray is unable to pray aloud, someone else may be chazan for him, say the Kaddish and Barchu for him, and start the first three berachot of the Amidah out loud. Afterwards, that chazan who prayed for him must finish the rest of his prayer silently (see Rama 69 and Mishnah Berurah 17).

^ 11.If six people start reciting the Amidah with the hope that four others will eventually join them, and indeed, after they conclude their silent prayer, four people who already prayed arrive and are willing to complete the minyan, according to the Radbaz, since when they prayed silently there was no minyan, they cannot recite the Amidah repetition. Many poskim maintain, however, that since they assembled in order to pray in a minyan, they hoped that people would arrive to complete their minyan, and they even comprised most of a minyan, so they may recite Chazarat HaShatz. So writes Har Tzvi Orach Chaim 1:51 and Yalkut Yosef 69:1.