Weclome to FlyerTalk, and sorry it's not under better circumstances.
This should be up to Expedia to resolve. They're your travel agent, you paid them, and if it was VA's fault, Expedia should still reimburse you and then seek payment from VA. But that's what should happen; it sounds like you've already gotten as far with Expedia as you can.
A credit card complaint would almost certainly get a refund of part or all of the original $600 ticket that was not honored, but that's not going to make you whole. The $3000 ticket was flown as purchased, so I don't know if the card company would help with that - though it's certainly worth a call to ask.
And you've already done a DOT complaint.
My only other idea would be to reach out to a consumer travel advocate to see if they could get someone's attention. I'm not a fan of Chris Elliot in general, but this seems like the sort of case he likes.