Best to discuss all with his parents.

You cannot merge the case but your father can apply for him under different visa category.

Yes this indicates that your case is under active review and should be scheduled for interview next.

Best to discuss with your retained counsel.

No the VAWA would be a completely new filing.

Yes sibling based petitions can take up to 16 years.

Unfortunately if there is no visa available based on the category of filing, expedited processing is not an option.

Difficult to say without understanding applicants eligibility.

The medical should likely be valid given validity date of 2 years.

Best to work with an experienced attorney for guidance.