According to the MB (122:5,6), it is better to cut short “elokaiy, netzor leshoni…” or even skip it, and then answer kedusha or kaddish or barchu, rather than to answer in the middle of “elokaiy, netzor,” even though we learned that that is permissible.
If one already said “yiheyu l’ratzon,” one should take 3 steps back and even try to say “oseh shalom…”
If one did not say “yiheyu l’ratzon” yet, it is better to say it now, and take 3 steps back (and try to say “oseh shalom”)
It is permissible to “use” a Shul for a mitzvah purpose–for example, it would be permissible to have R’ Yechiel Spira’s return kashrus lecture [using local food establishments as examples, taking place September 1 at 8:30 PM] in our Shul (as it is, though, it will iyh be downstairs–since even so, if possible, it should be avoided).
Financial calculations of a mitzvah (kuppah shel tzadaka) are also permissible, if necessary to do there. Poskim say that if those calculations are for the Shul itself (sweepstakes cheshbonos), it is lechatchilah permissible.