You want a sanity check in March?  I don't know if any of us would pass that.  😂

From the way you described it, it seems questionable if this separate "dwelling unit" is part of their Principal Residence or not.  I would say no.

*IF* it does qualify as part of their Principal Residence, the time from 2009 until it was converted to personal use would normally be "Nonqualified Use".  But I would need to think about if that would apply here or not.  But as I said before, my first inclination is that this separate "dwelling unit" would not qualify.