1- Sources indicate Imam Ali (a) was in a meeting with members of Bani Hashem when Lady Fatima (a) was suddenly attacked. When she was attacked he rushed to the scene, but she was already attacked. So it’s not like he was standing right next to her and he didn’t do anything. He was in a different room in a meeting.
2- As you said, the Prophet (s) had told him he would go through a tough trial after him and that he had to be patient for the sake of Islam. He fulfilled the Prophet’s request by being patient and not fighting.
3- We have too many sources that indicate her house was attacked. We cannot dismiss them. We have overwhelming evidence. Yes we don’t have exactly what happened and how the events exactly unfolded, but we do know an attack happened on her house.
You can see of the sources here: