Discoverer Aggregation / Analytics?

I'm attempting to use Discoverer to create a rolling 12-month attrition report. It works fine, up to the point of trying to create a average headcount for each month in the current 12-month period over the last 12 months' each. The problem I'm encountering involves the use of the MIN() function in selecting active employees in each month, mostly due to a data-cleansing issue, which I'd hoped to bypass.

Because some invididuals have two "data conversion" records - i.e., they were converted to the new database and an additional, subsequent record re-used what should have been a unique action reason - I need to test their MIN(position start date) so as to then use their actual start against the first record, whereas I can go on to use their position start to capture their FTE for any subsequent active records.

So I created a calculation, Min Start, to hold the earliest start date for each employee:

MIN(Position.Start.Date) OVER (PARTITION BY Employee.Number ORDER BY Position.Start.Date RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)

Then I test the status of each position for each month in the report and return the FTE where it tests as active through month-end.

CASE WHEN Position.Action.Reason = 'CNV' THEN
CASE WHEN Position.Start.Date = Min Start THEN
CASE WHEN Hire.Date <= Month.End THEN
CASE WHEN Termination.Date is NULL THEN FTE

[code]...

However, I can't SUM this because nesting isn't permitted and I can't average the sum because Aggregation of Analytic functions is not allowed either. I need a different approach. Data is always going to be dirty, so coding to account for such problems means I can perform my reporting requirements without interruption for clean-ups. (I know that keeping the data clean is best and highlighting such problems brings the attention of managers and staff to rectifying and avoiding such problems, but I still need to get the results out.)