I am looking for explicit mention in KMG Mahabharata that Karna learnt vedas from Drona (will be helpful if there are passages that show he conducted vedic rites). I don't think he learnt vedas from Parshurama but I might be mistaken.
The reason being Manusmriti and other dharma shastras seem to indicate that children born out of Pratiloma marriages (e.g. Suta) are not eligible to learn vedas. So any reference will be helpful .
I came across the following in Udyoga Parva
O son of Radha, thou hast worshipped many Brahmanas fully conversant with the Vedas. With concentrated attention and mind free from envy thou hast also (on many an occasion) enquired of them after truth. Thou knowest, therefore, O Karna, what the eternal saying of the Vedas are.
Seems that he had knowledge of the vedas after all. But does it mean sutas can learn vedas? Even Vayu Purana seem to say otherwise verse 28-29