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.