The following is all inferred by the koi based on the plot; after all, even that crafty old creator Kishimoto didn't specify it clearly, and there are numerous bugs in the Naruto timeline. The author is merely attempting to reasonably deduce the timeline.

[First, let’s clarify one point: according to the official materials I found, Kushina became the new Nine-Tails Jinchuriki as an adult, just as the first Nine-Tails Jinchuriki, Mito Uzumaki, was nearing the end of her life. For strategic needs and to maintain the balance of the tailed beasts, Konoha selected Kushina as the new Jinchuriki.]

There are several key phrases here: [as an adult], [for the sake of... Konoha selected]. From these keywords, we can conclude that Kushina truly became a Jinchuriki only as an adult, and secondly, it was Konoha that made the selection! This means that in the final decision, Kushina was merely one of the candidates. After all, in such a large village, how could they prepare only one container?

...

Next, let’s continue. Uzumaki Kushina was brought to Konoha at the age of six or seven. So, first we must address a question: who has the right to choose the Nine-Tails Jinchuriki, the Senju clan’s Mito, or the Third Hokage’s regime? (Of course, this is considered a need for the storyline of this book, so let’s boldly speculate.)

Secondly, is it truly impossible for the Jinchuriki to be anything other than the Uzumaki bloodline? The Jinchuriki in the five great ninja villages, apart from Konoha, almost all have their own methods.

Thirdly, there are too many bugs in Kushina’s growth story, especially regarding the hero saving the beauty with Minato. (This book makes reasonable bold deductions.)

During her time in the ninja academy, she did not become a Jinchuriki. After all, when she was captured by the Cloud ninja, only little Naruto went to save her. If she had already been the Nine-Tails Jinchuriki at that time, the village would certainly have increased their efforts. One could argue there was a conspiracy, but what if the Nine-Tails' power had erupted? Wouldn’t they be afraid of losing control?

Moreover, according to the manga, one becomes a Jinchuriki as an [adult], but the timeline of Naruto is quite chaotic, and the specific time cannot be determined.

Therefore, based on the plots from the anime and manga, along with the settings of this book, the author boldly speculates to arrive at the current storyline. The author is also boldly speculating; even Kishimoto himself hasn’t made it clear, so don’t take it too seriously. If you do, the author can only point out the manga's setting that one can only seal the Nine-Tails as an adult.

... The following is the storyline that the author has boldly speculated, which aims to be reasonable, and I hope everyone can accept it.

Politics is the art of compromise; according to the conclusions drawn by the author’s reasoning:

First: As one of the candidates, Kushina was chosen by the Senju clan, and similarly, there should be other candidates within the Third Hokage’s village.

Second: There are too many instances of the hero saving the beauty that lack foundation, but there is one point: Kushina’s heart is set on Minato Namikaze, which serves as a bridge between the Senju and the Third Hokage’s faction. Minato is part of the Third’s faction and is also the inheritor of the Will of Fire. The emotional bond between the two led both sides to choose compromise and acceptance.

...

All of the above are bold deductions made by the author based on the plot. If there are any specific bugs, please overlook them. Every book has its own settings and should not be rigidly applied, right? After all, that crafty old creator Kishimoto has left many holes, and the timeline is quite messy.

I hope everyone can understand. Really, the author has spent a lot of time organizing this timeline, and for those who like it, please vote in support.

Thank you!