Realm mergers as stated in the rules and policies does not, as originally envisioned, refer to a 'push button to join' another realm but through the welcome and friendly take over of all the regions of a realm which is not allowed. Realm mergers are only allowed if all its regions are taken over through war. This would be the meaning of 'no friendly realm mergers allowed'. Also, previously, the last duchy (with the capital) could not join another realm. This was regulated through game mechanics.
It is currently through a bug that realm mergers with the last duchy containing the capital can join another realm by 'pushing a button'. Even so, any merger of an entire realm by its original conception whether it is by by pushing a button or through the friendly takeover of all its regions outside of a war is not allowed.
