I can sympathise - trying to work out the names and identifications of some groups is very tricky and particularly so when we have cultivated and "improved" things through selective breeding and hybridisation. I think it most likely that instead of your Viola being pure V. cornuta L. the Horned Pansy, it is much more likely that it is a hybrid between this and the Garden Pansy Viola x wittrockiana - which as the "x" denotes is itself believed to be of hybrid origin derived from crosses between the Field Pansy (V. arvensis) and Wild Pansy (V. tricolor). Most Bedding Violas and Violettas are of this nature.
I was fearing it was in deed a hybrid but I think I'll keep it in my herbarium anyway. It's so beautiful! And yes, I find this group particularly tricky as well. I haven't studied deeply many more plant groups, but violas and violettas have many very similar species, in my opinion.
Anyway, thanks a lot for your kind and complete answer!