I saw one report on reddit where someone claimed Chase gave them back the full refund. But the vast majority of reports from Chase customers were that they were told it's too late to open a dispute (as I was told too). If that poster truly gets to keep his/her full refund, well done to them. I didn't have the energy to gamble on a chargeback working, and then missing the nonsense window to request the 80% refund.
I think you've done the right thing, as distasteful as it feels. As soon as you have the partial refund I'd send a letter requesting/demanding the rest, and then make chargeback attempts on that portion, consider small claims, etc. I wouldn't feel morally bound to this new "waiver" that was signed under duress -- legally it might make it a bit more difficult later, but it was going to be hard anyway to get the full refund, so nothing to lose in trying.
Hopefully future olympics will see drastically reduced ticket purchases as consumers remember what happened here (social media can be a great way for bitter victims of this one to keep bringing it up in the future). That's probably wishful thinking though.