When we say that a person must stop reading the 'Amidah and concentrate on the Qaddish and Qedusha if it is being recited by the Hazzan, this only applies if the main Shaliyah Sibbur (prayer leader) is the Hazzan. The reason is that a permanent Hazzan understands the Halakha that he must have the intent (Kawwanah) that those listening to him also fulfill their obligation through his prayer (provided they have the same intent).
Someone who just happens to be the Hazzan on that occasion, very likely does not know that he needs to have this concentration. As such, no benefit is achieved by pausing the 'Amidah and concentrating on his reading. In such a case, even if one is near him and can clearly hear every word he is saying, one does not need to pause, but can continue to read the 'Amidah while the Hazzan is reciting the Qaddish or Qedusha.
The situation is different, however, if the person who is the Hazzan, is a Talmid Hakham and Yerei Shamayim (G-d fearing Torah scholar), and understands that he must have the intent to allow the others to fulfill their obligation through him. In this case, even if he is not the main Shaliyah Sibbur, one must pause to hear his Qaddish and Qedusha, in the manner we mentioned previously (see http://www.atorahminute.com/2017-12-04 ).
(see 'Od Yoseph Hai, Mishpatim, Oth 6)
Od Yosef Chai, Rabbi Yosef Chaim from Baghdad, Kaddish Kedusha