Just to follow up...after 3 weeks and 2 separate Oxy10 treatments, spaced between a week, the stains were faint but not completely gone.
I didn't wait any longer, and glued the edges of all the black pleather items with plastic wrap, trimming the edges of the wrap accordingly. The glue to use should be any rubbery shoe glue type, but I ran out of that, and simply used UHU glue, which really isn't appropriate because it hardens. Any way, it works for now for the stain shielding job, as shown.
I did the same for the pleather sleeves. The body under these outfit pieces are of course wrapped in plastic wrap.
After a month, the stain is pretty much gone now.
Subsequently, all these others, ie. Yuna and Yuffie, have the same stain protection treatment to the black clothing pieces, plus body wrapping under them respectively. Tiffa's new stockings and black tank-top were socked in Dylon S.O.S Colour run for an 1 hour prior to use. This fabric stain remover works really well...much better and faster than vinegar!