Right after borochu in shacharit there is a bracha “yotzer or uvoreh choshech, oseh shalom uvoreh et ha kol”. Should this bracha be said out loud by the shaliach tzibur? Regardless of whether it should or should not, if it is said out loud should amen be answered at the end?
Yes, the Chazzan must say that line out loud. In fact, as part of his job as Chazzan, he should really recite all the blessings surrounding the recitation of Shema, in the morning and the evening, aloud in order to fulfill anyone's obligation. However, R Yosef Eliyahu Henkin writes that at the very least he must recite the closing of each blessing, a small thematically essential part of the blessing just prior to that, and the initial opening formulation, as those are the minimum required to form coherent blessings. Amen is only recited at the conclusion of each lengthy blessing (...Yotzer Hameorot; ....HaMaariv Aravim; ...Yisrael BeAhava; ...Ga'al Yisrael; ...Yisrael La'ad), not after the opening formulation, just like any other long blessing.
That phrase is the start of one long b'racha ending Baruch Ata Hashem, Yotzer Ham'orot, and not a b'racha of its own.
Therefore you should not say "Amen" at that point (after Et Ha Kol) but only at the end of the b'racha itself.
Look at the Mishnah B'rachot, chapter 2 where it refers to "breaks" in these prayers. http://www.emishnah.com/PDFs/Berakhot02.pdf
(1) If one was reading in the Torah [the section of the shema] when the time for its recital arrived, if he had the intention [of reciting the shema] he has performed his obligation and if [he did] not [have the intention] he has not [fulfilled his obligation]. In the breaks [between the sections] one may give a greeting out of respect [e.g., to his father or teacher] and return a greeting; in the middle [of a section] one may give a greeting out of fear [to a person he fears will harm him if he does not greet him] and return it; the opinion of Rabbi Meir. Rabbi Yehudah says: In the middle, one may give a greeting out of fear and return it out of respect; during the breaks, one may give a greeting out of respect and return a greeting to anyone. (2) The breaks are as follows: Between the first blessing yotseir hameorot and the second ahavah rabbah, between the second haboheir beamo Yisrael beahavah and shema, between shema and v'hayah im shamoa, between v'hayah im shamoa, and vayomer and between vayomer and emet v'yatziv. Rabbi Yehudah says, Between vayomer and emet v'yatziv one should not interrupt.
With regards to your first question: should the chazan recite it aloud? Yes, preferably, in case there is someone present who does not know how to recite the blessing, in which case hearing the opening phrase and the closing phrase, and reciting Amen at that point, may have heard enough to have "fulfilled" the blessing in a minimal way. (See the source provided by DoubleAA).