Q.  Kavod Harav,
(Re- question 1362 where Horav Shlomo Miller Shlit”a ruled that once one has started the amida and said the first words of the bracha he should continue with the rest. The reason is that it is improper and disrespectful to interrupt the address being said to the King of the Universe).
Would the rav rule differently if the person merely said Hashem Sefasai etc. but did not begin the actual Amidah (Baruch…). Ben Ish Chai (1st year, Vayakhel 10; Rav Pe’alim Vol. 4, no. 4 says that if a person has said the opening verse, and then encounters kaddish or kedushah, he should answer with the congregation, and then repeat the first pasuk again and begin davening. Mekor Chaim (122:1) writes similarly that one can make a hefsek for the purpose of answering kaddish and kedushah.

A. Although Mishna Berurah (66: 35) maintains that once Hashem sefassai was said it is considered as having already started amida, as you quote many Poskim rule that in mincha one can still answer. Ishei Yisroel (32: H. 47) seems to say that the Mishna Berura would agree. See also Piskei Teshuvos (111 n.14).
As mentioned, Horav Shlomo Miller’s Shlit’a opinion is that by mincha, only when the and the first two words of the bracha were said one should continue and not be mafsik.

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