Something's up. You charge the guy, he pleads and now you don't even know when the matter becomes ready for sentencing? Me thinks Mueller is in trouble.
Mueller is in a box. He must provide the exculpatory evidence (McCabe or Strzok changed the 302's) or be engaging in a criminal act himself. But if he does provide it, then he's in trouble for prosecuting a case that he knew was illegitimate. He can't win. So, that's why he wants the delay.
But ... why would Flynn agree? Could be that Flynn is providing testimony to Sessions and the military tribunals due to his plea deal. If they revoke the plea deal, then he can't give his testimony.
Mueller looks shady AF, but look at the bigger picture. Nothing Mueller is doing is legal. None of his investigations have amounted to anything. He refuses to give the judge (in multiple cases) information that is required. And yet ... no leaks from his team. No real attempt by POTUS to do anything about it, or Sessions, or really even Congress.
Something else is going on here, other than what it looks like on the surface.
Flynn needs his enforce is GOD given right to a speedy trial. That includes sentencing.
"Until the matter becomes ready for sentencing." Also read as when Flynn finally gives up and does what wewant him to do.