Maybe Sasuke teaches him the Chidori in a moment of camaraderie. And for the Rasengan, it could be an older and wiser Naruto from the future who travels back in time to teach his younger self.
In some fanfictions, Naruto might learn Rasengan through the guidance of an alternative mentor. Maybe a powerful shinobi from another village takes him under their wing and shows him different techniques to master the Rasengan. This could add a whole new dynamic to the story and his character development.
No, Boruto is alive in the manga. The plot hasn't led to his death. However, the manga is ongoing, and anything could happen in the later developments.
It could be Kakashi. Since he's already Naruto's teacher and he knows both techniques. He could guide Naruto in understanding the intricacies of the Chidori, like how to form the lightning - chakra in his hand, and for the Rasengan, he could give more advanced tips on chakra control.
In a fanfiction like this, the plot might involve Naruto struggling to master both techniques. He could start by observing Sasuke use the Chidori and then try to incorporate its lightning - based power into his own jutsu. For the Rasengan, he might have new insights while training, perhaps finding a way to combine it with the Chidori for an even more powerful attack. Along the way, he would face challenges like controlling the chakra flow for both techniques and dealing with opponents who are also after these powerful jutsus.
Another plot device could be a mind - link or a shared vision. Maybe Naruto has a moment where he links minds with a past Hokage who was an expert in Rasengan. Through this connection, he gains instant knowledge on how to form and use it. It's a more supernatural way of learning compared to the traditional training.
Yes, Boruto is indeed a manga. It expands on the Naruto universe and offers fresh perspectives and storylines. Many fans enjoy reading it to delve deeper into the world created by the series.