May one answer Amen etc if he is in range of hearing a Minyan that does not follow proper protocol for Covid-19, or is taking place contrary to the instructions of the local medical or/and Rabbinical directives?
It is forbidden for a Chazan to lead a Minyan against the will of the congregation, and if he does so, it is forbidden for one to answer Amen to his blessings. Minyanim which are taking place contrary to the local medical and Rabbinical directives, are similar in law to a Chazan which is Davening against the will of the congregation, and therefore it is not only forbidden to join such Minyanim, but one is not allowed to answer Amen towards their blessings even if he is within hearing distance from inside his home. The same applies to answering Kedusha, Barchu, Kerias Hatorah, and any matter of the like.
 Admur 53:29 “One may not pray against the will of the congregation. Whoever Davens against the will of the congregation, out of forcefulness and haughtiness, one may not answer Amen after his blessings.”; Rama 53:22; M”A 53:26; Aggudah Brachos Perek Keizad Mevarchim, brought in Binyamon Zeev 163; Kitzur SHU”A 15:12
The reason: As the verse [Tehillim 10:3] states “Ubotzeia Bireich Noeitz Hashem,” that Hashem hates one who says a blessing on stolen bread. [Admur ibid; Binyamon Zev ibid] See also Admur 11:12 regarding stolen Tzitzis, Admur 586:4 regarding a stolen Shofar. Accordingly, even if the Minyan is taking place due to deep religious reasons and Mesirus Nefesh, nonetheless, since according to Halacha it is forbidden to take place due to the medical danger, therefore it is similar to a Minyan which is stealing from the public, and therefore a blessing is not to be recited.