Khelben was a Chosen, after all... If he had truly been rendered helpless just because his opponent's spell resistance was high enough and skin thick enough, he would have been killed countless times already.
Even though he had expended a bit of effort because he was reluctant to resort to violent methods, he still managed to bind this half-elf who also hailed from Myth Drannor... He had only studied at that Inkastam Academy during his youth, and though those days were not particularly pleasant, he still could not bring himself to kill a half-elf who knew of that past.
He was not truly nameless or rootless... That period back then when he had to live under an alias, unable to even use the name "Orosan"—the only proof that he was Olang's son—was the sorrow he least wished to recall.
Furthermore, Khelben needed to figure out why his former companions from Myth Drannor hated him so much.
Although he had acted a bit too excessively when he broke ties with the elves back then, he knew very well that given the magnanimity of Corellon, the Father of Elves could care less about even Fenmarel Mestarine, the wayward god who had once almost betrayed him alongside Rose, let alone him. He, Khelben, had merely acted a bit radical.
Therefore, as long as he successfully became a demigod—and it would be best if he formally ascended... Azuth only needed to divest a fraction of his divine portfolio for him to use, and magic-related portfolios were always powerful, otherwise Vashar could not have broken away from Talos so easily.
At that time, he could simply express his remorse to the Elven Pantheon.
As for Mystra... He understood Midnight's personality very well. Even if she regretted promoting him, she would at most ignore him, much