The idea of a chazan starting chazarat hashatz without first reciting a silent Shemoneh Esrei is discussed in the Shulchan Aruch,[1] based on the Kol Bo.[2] In short, it can be done, but it is not ideal. Understanding why it is not ideal will help guide us as to how to proceed in different circumstances.
The Mishna Berura[3] assumes that when the Shulchan Aruch permits the chazan to recite chazarat hashatz without a previous silent Shemoneh Esrei, he refers specifically to a case in which no one else is capable of serving as chazan. The Mishna Berura does not state why it should not be done otherwise.
The Kol Bo identified two issues in this context that we must address:
1) If this chazan is reciting Shemoneh Esrei for others (the tzibbur and/or one who cannot daven himself), how does the chazan fulfill his own personal obligation? The Kol Bo maintains that if the chazan’s tefilla helps others, it certainly works for himself; he therefore does not need to repeat Shemoneh Esrei after his chazarat hashatz. Despite this strong logic, the Mishna Berura may imply that this is true only after the fact (b’dieved); it is better for the chazan to daven his own Shemoneh Esrei for himself, independent of chazarat hashatz.
2) How can the tefilla that is also serving as a personal Shemoneh Esrei be recited out loud? Reciting a personal Shemoneh Esrei audibly is usually forbidden, because it makes the davener appear as though he does not believe that HaShem hears silent prayers.[4] The Kol Bo answers that this is not a problem in this case, because the chazan is doing so due to pressing circumstances. However, the Eliya Rabba[5] writes that this issue is enough reason for the chazan to avoid using the chazarat hashatz as his personal Shemoneh Esrei if better alternatives exist.
The Magen Avraham[6] writes that the main problem here is based on the gemara in Rosh Hashana.[7] The gemara states that the reason to have a silent Shemoneh Esrei even though people can fulfill the mitzva through chazarat hashatz is to give the chazan an opportunity to familiarize himself with what he will be reciting. Although this is not crucial for a standard tefilla,[8] it is still a reason to prefer that the chazan not skip his silent Shemoneh Esrei and simply start with chazarat hashatz.
Is switching to the late-coming chazan justified because he is an avel? We did not find classical sources on this question. Among less-known Acharonim, Birkat HaBayit[9] allows this practice within shloshim and on a yahrtzeit, but not during the rest of the year of aveilut. Ishei Yisrael[10] writes that any chiyuv[11] of aveilut suffices.
Who should be the one to decide which opinion to follow? According to the Eliya Rabba, this is a question that relates to the propriety of the chazan’s personal tefilla (out loud), and it is thus up to him (or his posek) to decide. According to the Magen Avraham, however, the issue is the chazarat hashatz’s quality (without preparation), which is the minyan’s call.[12] Certainly, on a matter that is about no more than preference, we would not want the issue to cause discord, which we consistently urge to avoid when possible.
We surmise that your story occurred at Mincha (i.e., the avel was only a few minutes late), and this is the Kol Bo’s context as well. In this case, permitting the late-coming chazan to go straight into Shemoneh Esrei, preferably after reciting Ashrei, is straightforward. Regarding Shacharit, which is usually preceded by P’sukei D’Zimra, Birchot Kri’at Shema, and Kri’at Shema itself, if he was able to get up to Ga’al Yisrael on time, it is possible to do the same thing as we discussed regarding Mincha. Otherwise, there is additional reason to object to his serving as chazan, as it is problematic to start Shemoneh Esrei without the sections of tefilla that lead up to it.[13]
Regarding your question of continuing out loud after Kedusha, the sources clearly assume that the chazan will be doing so, as he “fills the shoes” of the chazan for chazarat hashatz. In any case, he has no justification to drag the minyan into a less than ideal chazarat hashatz (known as Heiche Kedusha) or worse.[14] After all, there is not even an obligation to let an avel be chazan;[15] assigning him Kaddish is enough. Therefore, once the avel has started chazarat hashatz, he definitely should not revert to “silent mode.”