Was Surya so powerful in the Mahabharata even Nara (Arjuna) and Narayana (Krishna) together could not defeat his son when he had all his power?
Yes, the answer coming from Krishna himself.:
Krishna says to Arjuna in the Critical Edition.:
It is through good fortune that his armour has gone. It is through good fortune that his earrings have been robbed. It is through good fortune that the invincible spear has been used up on Ghatotkacha. Had he possessed the armour and had he possessed the earrings, the powerful Karna would have been able to defeat everyone in the three worlds, even the immortals. Vasava, Kubera, Varuna, lord of the waters, and Yama would not have ventured against Karna in an encounter. Had that bull among men possessed those, you with your Gandiva and I with my sudarshana chakra would not have had the capacity to defeat him in a battle.
Best explore Drona Parva and Karna Parva of Mahabharatam to solve all confusion. This is from Karna Parva.:
“Thus did that illustrious Deity (Mahadeva), that Creator of the worlds, that Lord of both the Gods and the Asuras, (viz Maheswara), did that which was for the good of all the worlds. As the illustrious Brahmā, the Creator of the worlds, the Grandsire, the Supreme Deity of unfading glory, acted as the driver of Rudra (during the destruction of the Tri-Purasuras), so do thou restrain the steeds of the high-souled son of Radha like Grandsire restraining those of Rudra. There is not the slightest doubt, O tiger among kings, that thou art superior to Krishna, to Karna, and to Phalguna. In battle, Karna is like Rudra, and thou art like Brahmā in the policy. United, ye two, therefore, are competent to vanquish my foes that are even like the Asuras. Let, O Salya, that be done speedily today by which this Karna, grinding the Pandava troops, may be able to slay Kunti's son owning white steeds and having Krishna for the driver of his car. Upon thee depend Karna, ourselves, our kingdom, and (our) victory in battle. Hold the reins, therefore, of the excellent steeds (of Karna)” ~ Mahabharatam Karna Parva 34.
Karna was a student of Parashurama. Drona was also a student of Parashurama. Karna directly learned from Parashurama. Whereas, Arjuna learned from Drona. Even Ahsvathama is also like Rudra. But Since Sri Krishna the Divine was next to Arjuna, he was saved.