I'm definitely not the expert you're probably looking for but have been using ArcFM for about 10 years now. The MM version always gets created as part of the posting process. If I remember correctly, the edited version is copied to it then it posts to default and then both the edit version and the MM version are deleted. Although sometimes they both hang around, hence the need for the Orphaned Cleanup tool in Catalog. Not sure about the problem you're having with the different posting processes....sorry.
This may not answer all of your question but it is a great explanation of the Orphaned Version Cleanup Tool (mentioned by Lane above) and what it does with these _MMPOST versions among other things:
I am totally in sync with Josh Pritt's explanation.
Though I am also not expert but using on Session Manager since 12 yrs hence will try to explain the same,
Whenever post button hits on session manager toolbar one temporary version gets created in background as SN_<number>_MMPOST (owned by SDE). Session manager actually posts this temporary version to parent ( default) version. Once this Temporary version gets posted. original version session SN_<number> marked as orphaned in SDE.Versions table as flag you can find it in "Hidden" column as value 0 or 1 and session gets deleted from session manager.
Further when "Orphaned Version Cleanup Tool" is used in ArcCatalog it reads this column and versions those are posted or deleted from session manager gets cleaned from system as no longer needed, later you can use compress option to move the edits to base table from delta ( A&D) tables.
In case of ESRI versions you need to manually delete versions after posting these versions does not get cleaned by FM Cleanup tool .
Hope this is useful.