Do other schools of thought besides the Maliki school obligate the tashahhud in the third rak'ah for the follower (which is the fourth for the Imam), and is the prayer of one who omits it valid?

The summary of the Fatwa:

We have mentioned to the questioner the school of thought whose adherents discuss this issue, clarifying that it is a narration within the school and not a saying of Imam Malik himself, but rather the view of his student Ibn al-Qasim.

We have discussed this issue in response to the questioner's request. However, our Fatwa is that whatever a worshipper catches of the Imam's prayer is considered its beginning. This is one of the matters of ijtihad in which a layperson is permitted to follow a qualified opinion.