MB 10: He should repeat - Because this verse is associated with the Amidah, as mentioned above. It is good to say it quietly. He need not say other verses but if he wants to he may. The Pri Megadim wrote that the Chazan may not interrupt [to speak] after finishing his silent prayer, if not for a Mitzvah-related issue, such as responding "Amen Yehei shmei rabah" after Kaddish [i.e. if there were another minyan in the building -- YM], but for an ordinary matter it would seem not.
Jonathan Chody jonathan@quantime.co.uk
111:3. One who has not yet recited Shema who encounters a Minyan that is [in the middle of] reciting the Amidah, should not recite the Amidah with the Minyan but should rather (11) recite Shema [first, and then] afterwards recite the Amidah, since adjoining Shema to the Amidah takes precedence [over reciting the Amidah with a Minyan].
MB 11: Recite Shema - Meaning with its blessings. This applies only to the morning prayers (Shacharis), but at night (Maariv) one should recite the Amidah with the Minyan and then recite Shema as written in 136:3.