Please post the table definitions for the relevant tables. What you're doing sounds way harder than this should be.

Isn't it as simple as DIVIDE(COUNTROWS('OrderLineItems'),COUNTROWS('Orders')) ?

This?
SELECT SortOrder, country, colour, amount
    , DENSE_RANK() OVER (ORDER BY SortOrder) AS rn
FROM #testing
ORDER BY SortOrder
        , Country
        , Colour
        , Amount

Oh wait,...

I wouldn't back up the structure to Access. There isn't a perfect 1:1 object mapping between the two. Why not just script out all the objects so you have all...

This returns the first of the current month
SELECT DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0) AS StartOfMonth

So you'd just add months to that.

Not sure if this will help at all, but here goes.
Say you create one query that calculates the Numerator (and you include the grouping column you need, so you...

In the outermost query (the first SELECT), you'd just do something like
SELECT [Aggregate1] / [Aggregate2]
FROM...

As long as the two values are correct in the source query,...

What did you try? A DATEDIFF to get an individual record's wait and then sum that?

Your logic explanation makes me think that maybe you want either ROW_NUMBER() or COUNT() with a windowing function.
SELECT ID
, Type
, ROW_NUMBER() OVER (PARTITION BY ID,...

Maybe Kenneth Fisher's article on CROSS APPLY would help, but without some sample data, it's hard to tell.

Something like this?

DECLARE @SearchDate DATE = GETDATE();
INSERT INTO MyTable (col1, col2, col3)
SELECT col2, col2, DATEADD(day,1,@SearchDate)
FROM MyTable
WHERE col3 = @SearchDate;

Here's some actually tested...