OK, I figured it out.

Embarassingly simple, if you use set analysis and set dimentions to ignore to {$<dimension=>} then Qlik ignores selections for that, so the final formula is, in my case:

sum({$<[Marque Description]=,[56Marques]=,[Filtered Brand]=,CATEGORY=>}  [Depletion9L R12])

hope this helps someone else, as much as I loved learning the intricacies of nested ifs and getfieldselection use in set analysis, would have saved a lot of screen time just having to type out this one liner.