So on the Retained Earnings piece - is the account tagged as an Asset in EBS?
After you close December (assuming a Jan-Dec year) EBS should be sweeping any profit into Retained Earnings are move that into the starting bal of the next period. Does the balance you are looking at for RE in DEC incorporate the swept profit of the DEC P&L? I would guess that it is not?
Also - for period mapping - try creating one for Period 13 for a Year and pulling that in...
when data is extracted from EBS, it is stored in a staging table.
That table have fields for BEGIN_BALANCE_DR and BEGIN_BALANCE_CR
You may want to take the values from there