Don't think you need a new dataset. What you're describing sounds like you need a Matrix on your report.
Group on [Diagnosis Code], then at the intersection, do COUNT().
You...

Got Google?

http://sqlmag.com/t-sql/group-all

Did you look up the differences between DECIMAL, MONEY, and SMALLMONEY?

Did you look up the difference between SMALLMONEY and MONEY datatypes? MONEY should work fine.

Don't use BETWEEN with Times.  use <. > 
SELECT *
FROM MyTable
WHERE EventDate < @SomeTime AND EventDate>=DATEADD(minute,-10,@SomeTime)

What do you consider "unique messages"?  Do you mean something like "common sentences"?  I would probably use Jeff's splitter function and write the results to at least a temporary table...

SELECT TabA.Col1, TablA.Col2, TabA.Col3, TabB.X, TabB.Y
FROM TabA CROSS JOIN TabB

What do you mean by "missing days"?

Explain the logic of how to do it. I don't think it can be done definitively. Say you have a total for two invoices on the same date one is...

Did you update stats after the upgrade?

sp_depends

https://docs.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/sp-depends-transact-sql

Something like this?:
CREATE TABLE Test(BadData varchar(15));
GO
INSERT INTO Test(BadData) VALUES
('Data0 [2]'), ('Data1 [2602]'),('Data2 [130]');

SELECT BadData
    , LTRIM(LEFT(BadData, CHARINDEX('[',BadData,1)-1)) AS Loc
FROM Test;

SELECT ... NULL AS ColumnName
FROM MyTable
doesn't work?  Got some dummy data we can play with?

If the two result sets have a 1:1 relationship, use a JOIN.  If the two sets do not overlap, use UNION.