Unknown should be used, if maiden name is unknown and only the married name is known.
Proposing a merge just sets the status, edits, etc can be done on both profiles until the merge is completed. If you select to do the merge after approvals are done, you should always 'compare again' the profiles to make sure they still are the same person. Frequently, changes are made so they no longer match. I had proposed a merge between 2 profiles that were the same, but the PM changed both profiles with dates, parents, etc, and then rejected the merge saying they had different parents!! Of course, that is not what they looked like when the proposal was made, but that is part of why there is a wait time before the Proposed Merge will get default approval, also.