OP - In your many emails from SV, do you have an e-ticket receipt which has an e-ticket number? That is the starting point.

If you don't have an e-ticket receipt, you never had a ticket and nobody ripped you off. On the one hand, SV screwed up in failing to issue an e-ticket, but on the other hand, over 8 months you never confirmed that you had the basic travel document to board your flight.

A further suggestion here is that it is likely that you did not use DL's online check-in (available 24 hours in advance). One of the many reasons to do so is that when there is a problem, as there was here, you would have known in plenty of time to have had SV push the e-ticket to DL.