Hi Timothy,
You can use the SrmManagers from the SLSRMLibrary in combination with the ReservationInstanceExposers.
Can you try following code to retrieve the reservations that uses the resource:
var filter = ReservationInstanceExposers.ResourceIDsInReservationInstance.Contains(resource.ID);
var instances = SrmManagers.ResourceManager.GetReservationInstances(filter);

You can expand the filter to lock in the time frame of the reservations aswell
var filter = ReservationInstanceExposers.ResourceIDsInReservationInstance.Contains(resource.ID).AND(ReservationInstanceExposers.End.GreaterThanOrEqual(DateTime.Now)).AND(ReservationInstanceExposers.Start.LessThanOrEqual(DateTime.Now));