Name for me exactly when within the 20th century -- and with who -- the cover-up of the MMM finally ended within Mormonism?
The 1999 rebuilding and repair of the Mountain Meadows Monument by the LDS Church under the direction of Gordon B. Hinkley was a start, but not enough IMHO.
More recently, I believe that the Church's involvement in the 2007 150th anniversary of the horrific massacre and the official apology the Church gave there for members' involvement, and particularly the book by the LDS Church Historian, Turley, "Massacre at Mountain Meadows," which was published soon thereafter, have gone a long way towards that end.
Hopefully such actions and movement towards reconciling that terrible period in history (including the atrocities against LDS members in Missouri and Illinois which to date themselves have never been reconciled) will either start, or in this case, continue.