I wanted to ask if Jafar e Kadhab only lied about his Imamat for the sake of saving Imam’s life or was he actually a liar?

Some scholars from the pulpit claim that he indeed was under TAQIYYAH and was trying to SAVE Imam somewhat similar to what Muhammed Hanafiyah did.

I’ve also heard that the direct blood/child/offspring of Imam cannot be bad, all of the children of Imams were good.

Can you please clarify both sides whether he was indeed a liar or are there any sources that says he was trying to save the Imam.
(If possible please provide sources too)

There is no evidence that he used to practice Taqiyya to protect the Imam, peace be upon him. Rather, it is known that he was a deviant since the time of Imam Al-Askari, peace be upon him. There is no evidence for what they claim that the crusading son of the Imam cannot be a deviant.