This letter was written only seven years after the holocaust, when the spiritual [and physical] situation in Eretz Yisrael left much to be desired [that explains the first paragraph].
The reference to the gemara גדולה תשובה is that Hashem is מחזיר גרושתו so we see that we can be in fact divorced.
The answer is that there are levels of divorce and a stunning proof is brought from explicit gemaros.
