What leaps out right away are two things:

1. DECLARE @weekends INT (but this is never used, and doesn't appear to be required)

2. Is the equation giving the correct results?

Assume DATEDIFF(day,...

Unless I am missing something about the logical organization of the data, it seems to me that the problem stems from the inability to distinguish which eleven-week group each Wk#...

If I change the SPROC back to simply select the DueDate, then generate the DataSet from that, then change the SPROC to calculate the DueDate without regenerating the DataSet, the...

That was my thinking.  I have also tried "cast(isnull(duedate, dateadd(day, 7, startdate)) AS DATETIME) as enddate", but no joy there either. 

I'm not sure I can get .NET to display a date for...

I do this:

DECLARE

 @strSelectedCity  varchar(50)

SET

 @strSelectedCity = 'FairField'

SELECT

 XCode, City, CASE WHEN City = @strSelectedCity THEN 0 ELSE 1 END AS Sort

FROM

 City_Table

ORDER BY

Sort, XCode

Cheers.