I have been davening Shacharit before work in a shul that does the Heicha Kedusha on any weekday that they get a minyan. If there is no minyan, they do the Silent Amidah only. I can understand that because the "pressing time" is to finish at about 8:00 AM so people can get to work.
In this situation, I have been saying the first three blessings (including Kedusha) quietly along with the hazzan. This seems like the general practice there since no one throws in any "baruch hu varuch shmo"'s.
That would all make sense and seem self-consistent to me, but when the hazzan gets to l'dor v'dor, the congregants DO say "baruch hu varuch shmo" for that final blessing of the Kedusha (implying that they are not saying the blessing themselves), but then they do not say "Amen", they just roll on silently to Binah. I thought that someone should say "Amen" if they are not making the blessing themselves.
Is this mistaken? Some sources (http://torahmusings.com/2010/11/the-quick-mincha-ii-heicha-kedusha/ siting Biur Halacha 124:2; Aruch Hashulchan, OC 232:6.) state that we should actually go and restart the Amidah over from the beginning. Why is the restarting necessary if we said the first three brachot aloud already? Is this somehow related to not saying Amen? If so, the reasoning eludes me.
The same situation comes up for Musaf on Shabbat and Festivals (and maybe on Rosh Hodesh, I'm not sure).