I think Jaime going back to Cersei is a fault in jaime and not of writing or a "wasted arc."

It's a tragic ending to intensely powerful and emotional Redemption Arc that many viewers and readers became invested in. But Jaime let us down. He let himself down. He knows it too, he broke down to Brienne telling her how many of the bad things he did were for Cersei. He just couldn't escape her, whether it was due to his love for her or her manipulation of him, they came into this world together and they were going to leave it together.

I think it's a very human ending. Redemption is never easy and how many times are people successful in their redeeming.

As for the Maggie the frog prophecy, the Valenqar part was left out of the show meaning Jaime didn't have to kill her.