First, let us review your assumption that you may not step back. In general, you may not encroach within four amot[1] of the person behind you during the latter’s Shemoneh Esrei, even in order to take three steps back.[2] It is important to note that many poskim permit stepping backwards if the person davening is standing at an angle.[3] Many also permit an intrusion for the sake of a mitzva, which could include the ability to continue davening with the minyan. However, even if you cannot take three steps back at the end of Shemoneh Esrei because someone is davening behind you, your participation in the davening need not be significantly impaired, as we will discuss.
The gemara[4] identifies two factors that determine whether one has finished Shemoneh Esrei.[5] One factor is whether he has stepped back; the other is whether he usually says tachanunim (additional requests) at the end of Shemoneh Esrei. According to the version of the gemara that we accept, even if one has not yet stepped back, if he does not say tachanunim, then his Shemoneh Esrei is considered finished after completing the final beracha (Sim Shalom).[6] It follows that one who says tachanunim but has completed them is also finished, even before stepping back.[7] This is the situation you describe, and your question is valid. What can you do and what can you not do at that point, after completing Shemoneh Esrei but before stepping back?
The Shulchan Aruch[8] rules that after finishing the last beracha of Shemoneh Esrei, one still may not answer Kedusha until after saying, “Yiheyu l’ratzon…” The reason is that “Yiheyu l’ratzon…” completes Shemoneh Esrei, even though the actual berachot of Shemoneh Esrei have already been recited, just as “HaShem, sefatai tiftach…” opens it.[9] The Rama[10] points out that since Ashkenazim have the practice to say tachanunim (Elokai Netzor) before “Yiheyu l’ratzon…,” they can also answer Kedusha before it.[11] In any case, if one has said “Yiheyu l’ratzon…,” even if he is in the midst of tachanunim and thus has not stepped back, he can say anything that is permitted during Kri’at Shema.[12] This includes responding to Barchu and the main parts of Kaddish and Kedusha[13] and, for Ashkenazim, answering “amen” to HaKel HaKadosh and Shomei’ah Tefilla.[14] The reason to refrain from other worthwhile responses is that Elokai Netzor is somewhat connected to Shemoneh Esrei, although it is considered to be on a lower level.[15]
The gemara[16] compares taking three steps back after Shemoneh Esrei to taking leave from a king. (Davening Shemoneh Esrei is described as standing before the King.[17]) One should certainly feel limited in what he can do before taking leave of the King, even if he has technically finished Shemoneh Esrei and is permitted to respond under certain circumstances. Therefore, one should even skip parts of Elokai Netzor in order to avoid even responding to Kedusha before stepping back.[18] As we saw above, this does not warrant infringing upon another’s four amot. However, the Ma’amar Mordechai[19] intuits that if one is ready to step back and is prevented from doing so by a technical reason (e.g., someone is davening behind him), the level of standing before the King is reduced. In that case, he argues, one can even answer, “Baruch hu u’varuch shemo” upon hearing HaShem’s Name, even though this response is only a minhag. The Mishna Berura[20] and many other Acharonim accept this opinion, with some permitting participation in other parts of tefilla, such as Ashrei and Aleinu, if one cannot step back. The poskim discuss whether the same is true for religiously related utterances that are not directly associated with tefilla.[21] One may certainly read divrei Torah at that time,[22] and he may say the Tehillim he customarily recites at the end of davening,[23] but if possible, he should refrain from other positive speech unrelated to tefilla.