A notable example where Special Counsel was appointed and the existing narrative was in large part confirmed without huge rounds of new trials is actually fairly recent.

In 1999 AG Janet Reno appointed former Senator John Danforth to investigate the handling of the siege of the Branch Davidian compound in Waco Texas back in 1993. Specifically to examine serious charges that among other things that law enforcement had lit the fire that killed many inside, had fired first, and that the extensive use and presence of military equipment from the TX National Guard(and we also now know Special Operations community members including Delta) represented a violation of the Posse Comitatus Act.

And in essence the Danforth Report a year later found that none of it was true, with one notable caveat it should be said. For years the FBI's line had been that they couldn't have started the fire because they hadn't been using any pyrotechnic devices. That was untrue and had been obscured for years. A handful of Tear Gas grenades with small explosive charges had been used, but the report found little credibility in the idea of them causing the fire in the way they had been used and the timing. However there were a handful of internal disciplinary actions brought against some govt agents, specifically as it related to their less than forthrightness in the face of 1995 Congressional inquiries. And calls by the Texas Rangers to look at the actions of some individual ATF agents didn't really go anywhere.

We should also note that the report faced some stinging critique even at the time.

For example this piece from CATO: https://www.cato.org/publications/commentary/back-waco-waco-case-ought-reopen

Which Danforth refuted publicly: http://www.foxnews.com/story/2001/04/24/danforth-disputes-new-criticism-his-waco-report.html

Here is the full report in several PDF's: https://en.wikisource.org/wiki/Final_report_to_the_Deputy_Attorney_General_concerning_the_1993_confrontation_at_the_Mt._Carmel_Complex,_Waco_Texas

NB: While the report itself was released within the 20 year rule, the events it examined were years before, and in large part no other thorough exhaustive govt inquiry into the siege has happened since.