Q. If I Prematurely Step into (Mincha – Laud Kedusha) Shemoneh Esrei, but only say the two preliminary verses which precede the first bracha of Shemoneh Esrie, can I step out so I can respond to Kedusha of shiliach tzibour or am I committed to complete my Shemoneh Esrei? I.e. I had stepped into Shemoneh Esrie not knowing that a shortened version of Shemoneh Esrie was to be repeated, before the tzibour began their silent Shemoneh Esrei.

A. The Talmud (Brachos 4b) maintains that the addition of the posuk Hashem Sefosai Tiftach at the beginning of the amida is considered as an extended Shemone Esrei. Biur Halacha (110: 2) debates whether this verse is an actual integral part of the amida and if omitted, one has to repeat, or maybe it only means that saying this pasuk is not a hefsek that separates between Geulah and tefilah. He leans more to say that it is not really a part of the Shemone Esrei. Igrois Moshe (5: 24) quotes Magen Avraham and Lechem Chamudos (Brachos 6b) who assert that it is an actual part of the tefila, however he debates and mentions that Sheliach Tzibur recites this pasuk silently unlike the rest of the amida. It is apparent from Mishna Berura (67: 35) that once this posuk is recited, one already started the Ehemone Ezrei and you cannot be mafsik anymore.
Be’er Moishe (5: 50) maintains that once one has began saying this posuk he may not even take the three customary steps before the amida. However Mekor Chaim (122: 1) and Rav Poalim (4: 4) assert that in Mincha and Musaf, after reciting this posuk you may still answer Kedusha and Kadish, and then recite the posuk again before the beginning of the amida.
Even if you maintain that “Hashem Sefosai” is part of the amida, this is not be the case in regards to the pasuk “Ki Shem Hashem Ekra” added on Mincha and Musaf and you may answer Kedusha and Kadish. (Piskey Teshuvos 111: n. 15)
Horav Shlomo Miller Shlit’a recommends that when you find yourself in the situation you describe you should rather repeat silently the amida together with the Shaliach Tzibur. This will enable you to recite the Kedusha together with the congregation.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a