I contacted the Gettysburg NPS about the monument moratorium. I asked for a proper response that could be posted here. Plus an explanation for the post 1999 monuments (didn't know congressional action was required on the Louisiana monuments ). Their complete email:
Gettysburg National Military Park (GETT) could not approve the placement of any new commemorative markers, monuments or memorials in any location in the park because we have a policy that no new monuments or commemorate features shall be placed here.
A primary goal of GETT is to preserve the historic landscapes, buildings, monuments, structures, archeological sites, artifacts and archives that are significant to the outcome and commemoration of the Battle of Gettysburg. At Gettysburg, battle veterans themselves placed the majority of the existing monuments on the battlefield and in the cemetery between the 1870's and the 1930's. Due to the age of these monuments, and their personal connections with the veterans who survived the battle (and the war), these monuments are now considered to be historic in their own right. New monuments are consequently considered to be intrusions on the historic commemorative landscape of the battlefield, as designed and laid out by the veterans themselves.
You are correct that the 11th Mississippi monument, its advance position marker, and the Delaware monument were placed on the field after the 1999 moratorium. The National Park Service had provided final approval of the Delaware Memorial before the moratorium went into affect. The 11th Mississippi monument and marker was placed on the field as a result of congressional action. The National Park Service management policies state:
With regard to Civil War parks, new commemorative works will not be approved, except where specifically authorized by legislation. However, consideration may be given to proposals that would commemorate groups that were not allowed to be recognized during the commemorative period. (Section 9.6.2 NPS Management Policies. Download the document
here.)