Thank you for writing to us. My answers to your questions follow.
1. Please refer to Ṣaḥīḥ Ibn Ḥibān No: 6993, Sunan Timizī No: 3747, Sunan Ibn Mājah No: 133, Sunan Abū Dā'ūd No: 4649, Musnad Aḥmad No: 1631, Al-Mustadrak ‘alā al-Saḥīḥayn No: 5858, Al-Sunan Al-Kubrā No: 8195 and Musnad Abū Ya'lā No: 971.
2. The Shī‘īs believe that the divine guidance continues after the Prophet Muhammad (sws). Accordingly they believe in Imāms who according to them are infallible and appointed by God to guide people. The first Imām is Ali (ra), then Hassan (ra) then Hussain (ra) and then 9 other Imāms from the progeny of Hussain (ra). The last Imām (Muhammad Ibn Hassan - Mahdī), according to them, went to occultation in 329 AH and is still alive and will appear near the end of the world to bring justice to the world. Accordingly, just as the rest of Muslims have the books of Aḥādīth narrated from the Prophet (sws), Shī‘ī has books of Aḥādīth narrated from their Imāms. Also since Shī‘ī believe that their Imāms were appointed by God, they do not consider any other rule at the time of Imāms to be legitimate, unless it is ruled by the Imām himself or the ruler can be seen as the representative of the Imām.
3. Yes, Tabarrā is one of the ten essential branches of belief (Furog-i Deen) that the Shī‘īs hold. These are Prayer, Zakah, the Fast, Hajj, Khums, Jihad, Amr bi al-Ma‘rūf (advise to good), Nahy ‘an Al-Munkar (advise against evil), Tawallā (friendship with friends of God), Tabarrā (dissociation from the enemies of God).
4. Temporary marriage was practiced by Muslims for some time during the life of the Prophet (sws) and many believe this was a practice that was already in vogue among the Arabs even before Islam. According to Aḥādīth the Prophet (sws) later forbade people from it. According to the Qur’ān 70:29-31, the only ways by which a man can have relationship with a woman is either by proper (permanent contract) marriage or by owner slave relationship (which is not applicable at our time). According to the Shī‘īs, however, the verse 5:24 refers to temporary marriage. Non-Shī‘ī scholars disagree.
5. Purely from Islamic point of view, yes. They are both Muslims.