Where to release them should be the at the discretion of the police. There is plenty of open space in Arizona where releases can be made, much of it Federal range lands. No water or shade, but lots of space. No proof of status and no answer from ICE about their status? Deliver them to Federal "property". Around 12:00 noon each day. All this summer long. With a hearty "Congratulations, you're released" and a nod from the bus driver toward the direction south. They're on federal land so let the Feds take care of them.
Bus them to DC and release them at 1600 Pennsylvania Avenue.