2- We have hadiths that state showing excessive grief on Imam Hussain (a) is recommended. Some people consider tatbir as an excessive form of grief, so they do it to show love for the Prophet’s family. Tatbir is also a form of bloodletting which the Prophet did and is acceptable. So there is nothing haram about doing Tatbir.
3- Yes they had supernatural powers because they had full knowledge of the book, and per Sura Naml verse 40 the one who has some knowledge of the book can do some supernatural powers by Allah’s permission. Their knowledge of the unseen came from the Prophet (he passes it to them) and also from divine inspiration (just as Allah would send revelation to the mother of Musa (See Qasas 7) and Lady Maryam (see 3:42), Allah would also inspire the Imams .
4- We Shias believe that the one who knowingly rejects following the Ahlulbayt (a) only after the Prophet (s) may not have a chance in going to heaven. Secondly, not all Sunnis have said Shias will go to Hell. And why would they go to Hell? For criticizing some wives/companions that the Qur’an criticizes? Where does the Qur’an say if you criticize some of them or even hate them then you’ll go to Hell? Sunnis don’t have any evidence in saying we go to Hell. In fact, we are on the safer side. The Prophet said: I leave behind me the Qur’an and my family. If you hold on to them after me you shall never go astray. We hold on to the Qur’an and his family. We are saved
5- As for Imam Ali being added to the Adhan:
One: we can add mustahab phrases like “jalla jalaluhu” after saying la ilaha illa allah.
Or saying salawat after the Prophet’s name.
Two: mentioning the name of Imam Ali is recommended when mentioning the Prophet’s name. We have hadiths about that.
Three: many have opposed Imam Ali. Umayyads cursed him on their pulpits for 70 years. We say it to affirm his status which others have denied. The Prophet said “Oh Allah support whoever supports Ali” and so this is one way to support him.