It's not straightforward to say if the RWBY manga anthologies are canon. Some might be integrated into the overall canon of the RWBY universe based on their consistency with established plot and character developments. But others could be more like side stories or alternate takes that aren't considered part of the core canon.
The RWBY manga doesn't manage to fix everything that was off in the show. It does make some efforts, but it's not a comprehensive solution to all the show's shortcomings.
RWBY crossover RWBY fanfiction can enhance the universe by exploring new relationships. For example, characters who have never interacted much in the original series can form new bonds. This gives a deeper understanding of the characters' personalities. It can also expand on the lore. By mixing different elements from within the RWBY world, new aspects of the history, magic, and technology can be revealed.
Definitely not. The RWBY fandom is made up of fans who love the show for its unique world - building, interesting characters, and exciting storylines. Lemon fanfics with their inappropriate content can create a negative image of the fandom. It's much better to focus on fanfics that enhance the original concepts of RWBY, such as exploring new Semblances for the characters or creating new story arcs within the established RWBY universe without any sexual or adult - related content.
No. Nazi ideology is abhorrent and promoting it in fanfiction is completely unacceptable. RWBY is a series that generally promotes positive values like friendship, courage, and unity, and injecting Nazi themes goes against the spirit of the original work.