Nope. It was NOT a "backup plan". It was the plan from the beginning.
Regarding point 2: Gentiles were always allowed to join with Israel, but in Revelation and Isaiah (and elsewhere) we can see that the Gentile nations will serve Israel and will be blessed too.
Rev 21:24-25 (AKJV/PCE)
(21:24) And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honour into it. (21:25) And the gates of it shall not be shut at all by day: for there shall be no night there.
Isa 60:10-11 (AKJV/PCE)
(60:10) And the sons of strangers shall build up thy walls, and their kings shall minister unto thee: for in my wrath I smote thee, but in my favour have I had mercy on thee. (60:11) Therefore thy gates shall be open continually; they shall not be shut day nor night; that [men] may bring unto thee the forces of the Gentiles, and [that] their kings [may be] brought.
This what the kingdom of God will look like when it does come.
Regarding point 6: This aspect of God's plan is delayed per His knowledge and was not some sort of "surprise". The kingdom will be fulfilled, just like everything that God says. Both parts of His plan are the entire plan. So there is no "original plan" and a "backup plan". It was ALL the plan from the beginning.
So it's mostly correct, expect for your mistaken notions of "original plan" and "backup plan".