How do I do an outer join between all nested subqueries? (getting an error of 'OUTER' is not a valid input for this position)
You need to specify whether you want an left, right or full outer join. A left outer join is the same as left join. Assuming this is what you want:

[...]
FROM (SELECT subfactor.score, dateID, EmployeeID 
      FROM employeescore 
      LEFT JOIN subfactor 
          ON employeescore.SubFactorID = subfactor.SubFactorID 
      LEFT JOIN factor 
          ON subfactor.FactorID = factor.FactorID 
      WHERE DateID='Jan2015' 
        AND employeescore.EmployeeID='1' 
        AND subfactor.FactorID='A1'
     ) a 
LEFT JOIN (SELECT subfactor.score, DateID, EmployeeID 
           FROM employeescore 
           LEFT JOIN subfactor 
               ON employeescore.SubFactorID = subfactor.SubFactorID 
           LEFT JOIN factor 
               ON subfactor.FactorID = factor.FactorID 
           WHERE DateID='Feb2015' 
             AND employeescore.EmployeeID='1' 
             AND subfactor.FactorID='A1'
      )b 
          ON a.EmployeeID = b.EmployeeID
[...]