The hinged seat idea might work although it would make the space below far less usable for shoes, etc. I might opt for scrapping the existing vestigial bench altogether, sad as that may be. They could bring it back flush with the cabinets and either leave it walnut or else paint to match. Either way, I would make a couple of little bench stools that you can slide in and out as needed. They could be simple three sided inverted U shapes that are a little shorter and shallower than the cabinets and you can still stow shoes under them. Or just make two smaller stool cubes with a cutout hand hold that sit on either end.
You could also leave the walnut the way it is and make stools of some sort, although I don't love the look.
Oh, and bonus vocabulary and creative writing points for an excellent use of "vestigial."