I think there are two parts here. One part is to call the property and ask if they can waive the minimum stay requirement for this Platinum guest and their colleagues. The other part has to do with the conference room.

OP, did you try to do both in the same call? Maybe try to just deal with the min stay requirement. Once that is resolved, call the hotel sales dept about the conference room.

If you decide you don't want to stay without the conference room, then the whole process would need to be done through the hotel sales department.