If a legal notice is received on August 5th, requiring a response "within 15 days", what's the last day someone can file their reply?

You can challenge the passport seizure by filing a writ petition in the High Court, arguing that the impounding order is based on a settled case and is thus invalid. You can also argue that the seizure is disproportionate and a violation of your right to travel. The High Court can then review the impounding order and issue directions for its release if it finds merit in your arguments.