You can when that was the campaign promise of the election and you won the super majority to do so. Regardless of personal opinion, they did this the democratic way. We will see what the Supreme Court rules.
It is a violation of India's obligations under international law. Specifically, Part 3 of the International Covenant on Civil and Political Rights which India is a signatory of.
No, this is not a violation. It is a crime to enter India illegally, so detention cannot be considered arbitrary.
The single point of controversy is whether the Citizenship Amendment Act can omit a refugee on the basis of religion. The supreme court can issue a ruling on this and end the matter.