1- The hadith has not been translated accurately. The Arabic text doesn’t say he will “hear.” Rather it says an angel will call out, or that it will be “announced.” In this world, there is a barrier that prevents us from seeing and hearing the angels. The Prophet (s) in this hadith is informing us that when we repent this is what happens, and we know he is truthful. So he is giving us news of the unseen world. Yes, when we die and the barrier is lifted, we can see all these realities.
2- It seems that sometimes he had to be present in the mosque during Jama’a prayers. But there is no historical evidence that he prayed Jama’a. One hadith in the book Tafsir al-Qummi states Imam Ali (a) stood behind Abu Bakr “and prayer by himself.” This means Imam Ali (a) would pray individually and not Jama’a