The Shulchan Aruch (Orach Chaim 101:4) and Magen Avraham (s.k. 6) both write that the bakoshes written in Aramaic such as "Machnisei Rachamim" and "Ribbono Shel Olam", should be skipped out when one is davening alone. The reason is, an individual needs special help from malachim for his tefillah to go up to Shomayim, and malachim don’t help with tefillos said in Aramaic, as they don’t like this language (Taz and Magen Avraham).
An additional reason is that malachim don’t understand Aramaic (Kaf HaChaim, ois 26, in the name of the sefer HaTanya, siman 72). The Kaf HaChaim asks, that Klal Yisroel asks for their needs directly from the Ribbono Shel Olam without any middle person, so who cares if malachim don’t understand Aramaic?
However, as we mentioned above, individuals need special help from malachim to help their tefillos ascend, and if the malachim don’t understand how can they help.
The Selichos Mefurash of R' Yaakov Winegarten shlita brings from leading poskim that if the tzibbur is in the middle of selichos, even if they are up to "Ashamnu Bagadnu" after tachanun, an individual may still say bakoshes written in Aramaic, however, if one gets delayed and the chazon is already up to kaddish tiskabel, then it is too late, even if the individual started together with the tzibbur.
However, I saw brought down from R' Elyashiv zt”l that as long as there is still ten people in the shul, even if they are no longer saying selichos an individual can still say bakoshes written in Aramaic.
Being a Chazan for Selichos in Two Different Places
The Shevet HaLevi (5:66) was asked if a chazan is allowed to be the chazan for selichos a second time, for a minyan which has no chazan. The Shevet HaLevi writes, that it’s obvious that it is ok, and it’s like a regular chazon, where although he has already fulfilled his obligation, he is able to help others fulfill theirs. Especially nowadays, where everyone davens themselves and the chazon isn’t really doing anything anyway. Even the first time is really unnecessary, yet we still do it, therefore, there is no reason not to do it twice. All that will happen is, is that the chazan will end up offering extra praise to the Ribbono Shel Olam, which is perfectly ok.
