It is obligatory to help the Imam (peace be upon him) if he asks for help. He is also permitted to excuse his companions from fighting for him. He is the leader and can make such a decision as per the current circumstances. The reason for such a decision as it appears from the narrations is that Imam Hussein (peace be upon him) knew that he will be killed in the battlefield as the little number of his followers cannot protect him. Therefore, he gave them the choice of leaving him to face his destiny. This shows the high ethical position of the Imam (peace be upon him) and the great values that he possessed. Similarly, his companions were excused from undertaking the hardship of the fighting and facing certain death in the battlefield, and fighting was not obligatory on them, but they refused to take this opportunity and decided to fight and show the highest level of selflessness and sacrifice.
Our edicts are based on the jurisprudence of the Imams of the Ahlulbait and the narrations that reached us in authentic ways which all Muslims are required to act upon, since the Prophet (peace be upon him and his holy progeny) ordered the nation of Islam to refer to the Ahlulbait and follow them in the famous well-known narration that is reported by numerous ways in the books of Shias and Sunnis: "I leave among you the two weighty things, the book of Allah and my progeny; as long as you follow both of them, you will never be misguided".
Acting upon this narration and other narrations to follow the Ahlulbait is not obligatory on the Shias only, but it is an obligation on all Muslims.
Intentionally reciting the third testimony during the tashahhud makes the prayer void, since it is not amongst the divinely-authorized words that are allowed to be recited in the prayer. If he was unaware of the said ruling, his previous prayers are valid; also, they are valid if he recited it believing it to be recommended.