KK was not referring to Boruto's death causing the end of the world. He was referring to him being forced to use his Karma seal, and Momoshiki coming out
I tried to figure out the Tree Tails rebirth today, but the timeline is confusing. ( because of the databook and Zabuza) According to the databook, Kakashi graduated at age 5 and became a chunin at age 6, (confirmed chapter 35) while Rin and Obito graduated at age 9. And y'all were right the three tails came back with 1-3 period or shorter