Once you start the deletion the tool will start to delete the cards on the board, you'll be able to see how many cards have been deleted.
Please note: Once these cards have been deleted, there's no way to recover them.
No, only board admins can use this tool to delete the Archived cards on a board, if you're not a board admin you'll need to reach out to your board admin to get them to delete the archived cards on the board using this tool.
No, you'll need to delete the cards from any boards one at a time.
The tool will delete no more than 50 cards in a 10 second window, this is due to rate limiting within Trello. However, depending on the speed of your computer and your Internet Connection, this could take longer.