What is the ruling regarding someone who performed Hajj al-Tamattu' ignorantly concerning the intention for 'Umrah, and only intended for Hajj at the miqat? Is his Hajj valid?

If you entered into ihram for Hajj, then performed Umrah with the people after arriving in Mecca, your Hajj is valid, and this Umrah supersedes your Hajj. The fact that you did not initially intend Umrah from the Miqat does not matter, as the Prophet, peace and blessings be upon him, commanded his companions to convert Hajj into Umrah, except for those who had sacrificial animals with them.

The Hajj of one who performs the rituals of Hajj by following people without detailed knowledge of the pillars and obligations is valid and sufficient. However, it is better for the pilgrim to learn the rulings of the worship and to ask people of knowledge.