Not so sure, PracTac
The County is required to house the DA's office and they will be housed in a county building. The County is responsible for maintaining, cleaning, securing and providing utilities to the building. I believe it is clear that the County is the "custodian" of the building. The AG suggested, "To this end, it would be important that you, as District Attorney, have a clear understanding with Sandoval County regarding who is the “custodian” of the property for purposes of Section 30-14-1(C)." Stating that the DA is custodian of the building in some sort of agreement does not make it so.
The AG is suggesting that the DA make some sort of end run -- if the DA is the "custodian", then they can deny entry under trespass law. Were this to go to court, the DA's claim that he is the building's custodian, would not stand up to scrutiny. "Do you own the building?" -- "No". "Do you clean and maintain the building and grounds?" -- "No". "Are you responsible for utilities?" -- "No". "Then why do you claim to be custodian of the County building?" -- "Because, this agreement says so???". "Are you saying the County entered into an agreement with the DA, in order to regulate the right to keep and bear arms?" -- "Er, uh, no, no. I wouldn't say that".
OK, IANAL, but I think this is a pretty clear attempt to twist the law and end run the State Constitution. The entire opinion revolves around some unspecified agreement to grant "custodial powers" to the DA, even though it is clear, under New Mexico statutes, that the County is the custodian. Rant off.