Whose son is Kaartikeya?

According to Puranic Stories, Kaartikeya or Skanda is the son of Uma and Sankara.  However,  according to Mahabharata, which preceded all the Puranas, Kaartikeya is the sone of Agni.

The story of the birth of Kaartikeya or Skanda was described in 223-224 sections of Vana Parva of Mahabharata.  The Story is as follows:

Once the powerful celestial Rishis, Vasistha and others, having duly performed the ceremonies with the bright blazing fire, those great-minded persons offered oblations to the celestials. And the Adbhuta fire, that carrier of oblations, took them with him and made them over to the dwellers of heaven.

And while returning from that place, he observed the wives of those high-souled Rishis sleeping at their ease on their beds. And those ladies had a complexion beautiful like that of an altar of gold, spotless like moon-beams, resembling fiery flames and looking like blazing stars. 

The Adbhuta fire, thus transforming himself into a house-hold one, was highly gratified with seeing those gold-complexioned ladies and touching them with his flames. And influenced by their charms he dwelt there for a long time, giving them his heart and filled with an intense love for them. 

And baffled in all his efforts to win the hearts of those Brahmana ladies, and his own heart tortured by love, he retreated to a forest with the certain object of destroying himself. A little while before, Swaha, the daughter of Daksha, had bestowed her love on him. The excellent lady had been endeavouring for a long time to detect his weak moments; but that blameless lady did not succeed in finding out any weakness in the calm and collected fire-god. 

But now that the god had betaken himself to a forest, actually tortured by the pangs of love, she thought, 'As I too am distressed with love, I shall assume the guise of the wives of the seven Rishis, and in that disguise I shall seek the fire-god so smitten with their charms. This done, he will be gratified and my desire too will be satisfied.'"

That excellent lady (Swaha) at first assuming the disguise of Siva, the wife of Angiras (one of the seven Rishis),  sought the presence of Agni and pretended to be equally smitten by his charm.

Then Agni, filled with great joy and cohabited Swaha in the guise of Siva, and that lady joyfully cohabiting with him, held the semen virile in her hands.  'Then assuming the disguise of a winged creature, she went out of the forest and reached the White Mountain and quickly ascending a peak of those mountains, threw that semen into a golden lake. 

And then assuming successively the forms of the wives of the high-souled seven Rishis, she continued to dally with Agni. But on account of the great ascetic merit of Arundhati and her devotion to her husband (Vasishtha), she was unable to assume her form. Swaha on the first lunar day threw six times into that lake the semen of Agni. 

And thrown there, it produced a male child endowed with great power. And from the fact of its being regarded by the Rishis as cast off, the child born therefrom came to be called by the name of Skanda or Mahasena or Kaartikeya.

That boy had 6 faces, 12 eyes, 12 ears, 12 hands, 1 neck and 1 stomach.

This story that Skanda is the son of AGNI contradicts the story told in Bala Kanda of Srimad Ramayana and other Puranic story that Skanda is the son of Shiva.  The stories incorporated in Srimad Ramayana and other Puranas might be PRAKSHIPA, (insertions made at a latter date) by Shivaites.