What are the details of Doraemon?

Why did the Time Administration allow Doraemon to help Nobita? I actually want to ask this question in detail, because the more I think about it, the more horrible it is.

Doubt 1: Doraemon came to the past, which is absolutely not allowed by the future law. The time administration can definitely stop Doraemon from going back to the past. Question 2: Doraemon has so many props to change the law of causality, can everyone really buy them in the future? I don't think so. And according to Shi Xiu, the family is very poor, so why can we have so many props? Doubt 3: Doraemon has never had any evidence to prove that Shi Xiu is Nobita's great-grandson. I can generally think that the reason why the Time Administration made Doraemon return to the past is as follows: 1. Nobuo Nobuo is recognized as the future savior. 2. Doraemon's future contribution can get such privilege. (A series of actions by seven boys) But it's a bit far-fetched. 3. Contrary to 1, Xinfu is the future dictator, and Shi Xiu is not the great-grandson of Xinfu at all. Shi Xiu was sent to change the future. If the world line has not been changed, then Doraemon is the assassin who assassinated Nobita. Moreover, Nobita's universe is specially designed for Nobita, and no other future people can set foot in it. A story like terminator.