The adaptation of Saiyuki manga in Manga Rock is quite good. It manages to convey the main plot and characters effectively. But as with any adaptation, there are always some subjective opinions on how well it matches the original manga.
I don't know for sure. It depends on the licensing and distribution rights. You could try looking directly on Manga Rock or checking with other manga enthusiasts who might have the answer.
No, Manga Rock is an illegal platform and doesn't have the legitimate right to offer 'A Silent Voice' manga. It's best to look for legal options to read it.
Downloading Manga Rock is not a legal or advisable option. Many legal manga apps and websites offer a wide range of content for your enjoyment. For example, services like Shonen Jump and Crunchyroll have a great selection.