Hi there, for the files and folders deleted or lost on the MacBook Air, you're recommended to use iBoysoft Data Recovery Software which is designed with advanced drive scanning skills and data recovery algorithms. It scans your hard drives thoroughly for deleted or lost files quickly and securely.
While for the APFS hard drive not mounted in Finder issue, you can force mount it in Disk Utility manually. Launch Disk Utility > Select the hard drive > Click the Mount button on the top menu bar. For more solutions, please visit: External Hard Drive Not Mounting on Mac, How to Fix It?