Could the emails ever be recovered and by whom

Emails could be recovered from Clinton's server (if BleachBit were not optimally applied), Clinton's Blackberry, backups of the server, copies on the lawyers' laptops, from third-party servers, backups of the third-party servers, etc.

The FBI may have direct or indirect access to these systems. For example, they could work with the Senate's IT department.

See also How to Delete Secret Emails from Microsoft Exchange Server and my interview with Stuart Varney

That is why I use Protonmail. It is open-source (so no backdoors) and they do not keep backup copies or records on their servers (view the sourcecode or read their privacy policy). Your mailbox is also encrypted with end-to-end encryption support. You can also make your own keys and such for those really private emails you want to send (but the other user will need to know somehow what the key is and you need to say it under encryption channels only, otherwise someone may discover the key). Once a email is deleted off Protonmail, it is gone forever.