Though I'm not qualified to give you an authoritative answer as to WHY the Septuagint omits those parts, I do believe that the Hebrew text existed before the Septuagint. If the Septuagint translators omitted those parts, then the overall meaning still remains in tactintact - that God will restore Israel to Zion. God "multiplying Israel's children and blessing them generations beyond" is all over other scriptures
Genesis 22:17 "I will surely bless you and make your descendants as numerous as the stars in the sky and as the sand on the seashore. Your descendants will take possession of the cities of their enemies"
I will surely bless you and make your descendants as numerous as the stars in the sky and as the sand on the seashore. Your descendants will take possession of the cities of their enemies
Exodus 20:6 "showing love to a thousand generations of those who love me and keep my commandments."
showing love to a thousand generations of those who love me and keep my commandments.
and I rather assume it would apply to this passage even if it was omitted. Perhaps the Septuagint translators felt the same way and so they left it out on comfortable assumption?