Everybody knows that any agreement made under duress is null and void.
Apparently, not everybody knows that, FRiend...
That's true, but you could make the same statement about every single plea deal ever made in a criminal proceeding.
Flynn's case was seriously damaged not just when he pleaded guilty, but when he stood up TWICE -- in two different court hearings -- and testified under oath that the information in his plea agreement was accurate and truthful. In our legal system, if you have made an agreement under duress then a sentencing hearing before a judge or other tribunal is the time to make that clear.
I'd suggest that this chain of events is the REAL reason why the Federal judge in this case seems to be on a zealous crusade against Flynn. If the judge just accepts this new story line at face value, he's pretty much undermined the entire process of plea agreements in Federal cases dating back to the establishment of the Federal courts in 1790.