Touma will die.
But his soul will chose to go back in time, the the time when he was happiest, and he will be at season 1 first episode.
Mikoto who loves Touma want to be with him too and want to be in a time when Touma'd love her, but since he already loves Index she'll go back in time before Touma is born, and became his mother.
You need serious help anon
That's pretty fuck up. Wouldn't make sense how the LN goes anyway.