That good night: gift or punishment?

[Aiwendil]

There are some serious obstacles in accepting the version of the Sun and Moon as given in Myths Transformed.

Perhaps. But that seems to be quite beside the point; apparently Tolkien had the Myths Transformed chronology in mind when he wrote the Athrabeth. Whether this text (and Andreth's myth) could be reconciled with the flat earth version is a quite separate (though interesting) discussion.