Is the belief that the Imams can hear angels but cannot see them, not in conflict with the Qur'an's assertion that the religion has been completed and cannot be added to?
Hearing the angles is not restricted for the prophets only but many persons were mentioned in Quran used to hear the angles like mother of Musa ( We inspired to mother of Musa that: Suckle him ) Sura Al-Qassass, verse 7.
Mariam, the mother of Easa used to hear the angles (Sura 3m verse 42 and 45) but she was never a prophet. Sara, wife of Prophet Ebrahim was also hearing the angles as we read in Quran (Sura Hood, verse 69 to 73).
Even Sunni Muslims claim that Umar ibn Al-Khattab used to hear the angles. They called Umar ibn Al-Khattab by the name: Muhaddath Al-Ummah. Fadhaa'il Al-Sahabah, by Mohammad Hasan abdul-Ghaffar, V.4, page 6.
The 12 Unfallible Imams were granted by Allah great gifts and abilities to guide the Ummah to the right path after the Prophet Muhammad (SAWA).
Islam was completed before the departure of the Prophet from this life. Infallible Imams never added any thing but preserved the real teachings of Islam. Their status definitely higher than mother of Musa and mother of Easa and Sara wife of Ebrahim.who were hearing the angles.
Wassalam.