It's reasonably true. The movie captures many key elements and storylines from the comics, but there are some differences to make it work on the big screen.
Not entirely. While it borrows key elements and characters from the comics, there are significant plot differences and creative liberties taken to adapt it for the big screen.
Avengers: Endgame has both similarities and differences compared to the original comic book story. Some key events and characters are portrayed faithfully, but there are also creative additions and alterations to make it a unique cinematic experience.
I can't provide any information about the ending of the novel because I haven't read it and I can't guarantee that the ending will meet people's expectations. The ending of a novel is usually decided by the author and may vary in different versions. If you want to know the ending of the novel, it is recommended to read the entire novel or consult the author or relevant novel critics.
Endgame originated from the Marvel Comics universe. So, in a sense, it can be considered related to comic books. It brings those comic book elements to the big screen in an exciting way.