Double Dependent Drop Down + Blank Cells

It may help someone to better understand what you are trying to accomplish if you would attach a sample workbook that illustrates the issue.
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This is a low tech proposal.
Based on the drop downs in D18:E18, L3:L16 are populated using:

Formula:

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M3:M16 are populated similarly based on the drop downs in D19:E19
L17 is populated using: =SUMPRODUCT(--(L3:L15<>""))
M17 uses a similar formula.
Data validation for cell G18 has a source of: =OFFSET($L$3,0,0,L16)
The data validation for cell G19 is similar.
Let us know if you have any questions.

In the attached file the input (Lookups-Master) and output are on separate sheets.
On the input sheet the formula for the "Cleaned up list from table headers for building drop-down" formula is modified to read:

Formula:

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The modification allows the list to expand as new column headers are added to the table. Note that both highlighted lists may need to be moved depending on the number of columns anticipated.
On the output sheet the following formula produces the list for the dropdown in the yellow cells:

Formula:

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Note that the list produced by the formula is horizontal so it may be dragged down the sheet.
The source for data validation in the yellow cells is: =OFFSET($J2:$O2,,,,COUNTIF($J2:$O2,"?*"))
The source for the drop downs in column D is: =OFFSET('Lookups-Master'!$U$5,,,COUNTIF('Lookups-Master'!$U$4:$U$14,"?*")-1)
The reference of the OFFSET function is set to cell U5 to exclude the word Category from the list.
Let us know if you have any questions.