Here's my general understanding about University policy vs NPC vs campus Panhellenic vs GLO specific. A University Admin can make whatever policy it wishes and the sororities must abide by it because we are on campus at their invitation. The national organizations and/or NPC can attempt to negotiate with Admin, but in the end we are obligated to abide by the Univ's rules. Campus Panhellenic is required to abide by the Univ's rules and NPC's. If any of those are different than our own GLO's, we are obligated to follow our GLO policies (which should already be in line with NPC) unless we receive permission to follow the campuses. Does that make sense?
So in this case if it's the University that is saying dissaffiliation must be for the whole summer and not just the 30 days as stated in the Green Book, then there is not much that can be done aside from contacting your national org and/or national NPC delegate and hoping they can change Admin's mind. If it's your campus Panhellenic making this policy, then your national NPC delegate needs to work with your NPC Area Advisor to bring them in line with NPC policy.