The following may be of use to you.
The game works perfectly without the use of 'mercenary_unit' in EDU's attributes lines.
What I have done with my mod is have no units as 'mercenary_unit' as an attribute in EDU.
I have made ALL units recruitable by ALL factions in the 'ownership' lines of EDU.
The restrictions on who can recruit what unit can be set just in the buildings.
Units do not need to be categorized as 'mercenary_unit' to go in mercenary pools.
I have a separate mercenary pool for each Area, and they seem to work ok with any units you put in them.
The 'mercenary_unit' and 'ownership' lines just seem to cause potential conflicts between themselves and other recruitment rules.