shemoneh esrei - Why don't we say amen after a Heicha Kedusha?

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).

I have read through how does one do hechi kedusha? and When may one do a Hechi Kedushah? but this inconsistency still does not make sense to me.

Thanks!

Answer

I spoke to my Rav about this and he said that one starts over (as stated in the Aruch Hashulchan) for Mincha, but for Shacharit there is Smichat geulah l'tefilah, that is to say that one should continue straight from barchu through the end of the Amidah without a break. Therefore for shacharit you should say all aloud with the chazzan and not say amens, including for kedushah, but for Mincha you should say amen to exclude yourself and then repeat from the beginning of the avot blessing.