Thank you very much for taking out time and helping me with your detailed and satisfactory reply. Thank you very much for assuming my follow-up question and answering that too.
1.Do you mean that those Ahadith that are narrated by Imam Jafar Sadiq are sahih provided they are neither against the Quran nor defaming any of the Companions?
2.How and why Banu-Umayyah stopped people from narrating from Imam Jafar Sadiq and Imam Baqir? At this time I have a very soft and positive corner for Banu-Umayyah. I am not very much impressed by or attracted towards Banu-Abbas. So, please, help correct my line of reasoning if I am wrong.
3.At the time of the Prophet PBUH, Banu-Hashim was governing all kinds of political issues. How and why they were sidelined and Banu-Umayyah and then Banu-Abbas took the reign?
1- Any Hadith may be considered technically Sahih, if it fulfills the requirement of Rijaal evaluation (reliability of narrators) and Diraya evaluation (the content being inline with the Qur'an and known facts). However please note that when the source of the statement is not the prophet (pbuh) and is for instance Imam Sadiq, then even if the statement is considered technically Sahih, it only reveals the opinion of the individual (e.g. Imam Sadiq) and not the command of God or his prophet (pbuh).
2- Both Bani Umayya and Bani Abbas were kingdom systems rather than Khilafah system, meaning the power was always with one family and within one family. They were not like some of the European kingdoms today where the king has no political power. This used to make the system more towards dictatorship rather than a democrat one (with some periods of exceptions of course). Bani Umayya traditionally were always considered Bani Hashim as their rivals. This had made Bani Hashim less active and free at their time. At the time of Bani Abbas, obviously being from the same clan, at least at the start Alavies had more freedom.
3- I don't think it is correct to say that at the time of the prophet (pbuh) Bani Hashim were governing political issues. History reveals that the prophet (pbuh) used to look at fitness of the person for the job. One of the elements of this fitness was the tribe that the person belonged to (given the tribal system of Arabs). Where Bani Umayyah could get along better with people in certain areas, as reported, the prophet (pbuh) would send a person from Bani Umayyah to that land.
What brought Bani Umayya to the power was ignited by the conflict between Ali (ra) and Muawiyah. After the assassination of Ali (ra) Muawiyah came to the power and the Bani Umayyah kingdom started by him through his son Yazid. Later, for many political and social factors, Bani Umayyah were defeated and Bani Abbas took over.
Please note that all the above are based on my understanding of the historical records. One may agree or disagree with some reports and therefore end up with a different understanding