DB2 SQL: How to select all days in a date range even if no data exists for some days
You say you have a date table.  You don't give a name so I shall refer to it as DateTable and the column in it as TheDate.  You need to select from this table and LEFT OUTER JOIN to it.  That way all dates in the range will have a row in the output with NULL for rows where you don't have "real" data.  You can convert these to zero using COALESCE.  I'll use your first posted query as an example: 

Select 
    dd.date,
    dd.week,
    sum(COALESCE(p.policy_count, 0)) Policies
from DateTable dt  -- Your date table
left outer join magenta.policies p -- left join includes all dates and any polices that exist
    on p.trans_date = dt.TheDate
left outer join global.dim_date dd
    on dd.date = p.trans_date
-- use DateTable in the WHERE to get the full range of time.
Where dt.TheDate between (select week - 112 days from global.TODAY_DATE)
                  and (select week -   1 day from global.TODAY_DATE)
Group by
    dd.date,
    dd.week


Sorry, don't have a DB2 instance to hand to test it.  Forgive any typos I may have introduced.