Looks like what happens when an organization is dreadfully understaffed... and the person running it doesn't care.
I don't think this is staffing. Auditing 2 million cases would be a royal headache for any organization.
Rather, we are seeing typical government procedures at work. There's no
need to audit 2 million cases. Fix the fucking software then re-run the calculations for what people should have received. Send out checks based on that. You only need to audit the cases where people feel it's still wrong.
Lets look at another example--people who work while drawing Social Security. The government's IT processes crawl, every October my wife gets a letter from them saying that they are correcting her social security payment based on what she earned in the previous year. Her payment is adjusted and she gets an extra payment for the difference between what she should have gotten and what she did get. All automatic, no audits involved. That's how to handle something like this.