Private messages to other members do not initiate the merge process, and if we make mistakes in a merge proposal there are several opportunities for other members to correct any errors we made.
A merge proposal is just a proposal until it has been approved by members of both profiles' trusted lists, or until it goes to default approval after 30 days have passed. After a merge has been approved, a WikiTreer needs to do the merge, which is a process of comparing the contents of the two profiles and determining what elements should be included in the merged profile (it takes a human to make sure that the same contents aren't repeated in the biography because of a merge).
When we propose a merge, the other profile managers get a message that includes a "compare" link for the two profiles, so they can quickly see what you are proposing -- and reject the proposal if they disagree. Additionally, the merge proposal appears at the bottom of the profile, and any member who disagrees can reject the proposal -- even after it has gone to full approval.