Unique Values in Dynamic Down List

This may seem like a round about way to get this done, however...
1. Populate columns L:O for the four regions using: =COUNTIFS(OffersTable[[Offer]:[Offer]],$K9,OffersTable[[Region]:[Region]],L$8)
2. Populate lists of contents in columns G:J using: =IFERROR(INDEX($K$9:$K$22,AGGREGATE(15,6,(ROW($K$9:$K$22)-ROW($K$8))/(L$9:L$22>0),ROWS($A$1:$A1))),"")
3. Name (see name manager) the lists of contents using the following as the refers to: =Solutions!$I$9:$I$22
4. Use the following as the source of the data validation: =OFFSET(INDIRECT(G3),0,0,SUMPRODUCT(--(INDIRECT(G3)<>"")),1)
Let us know if you have any questions.