A crossgrain patch on the outside will stand out like a sore thumb. If this is a really expensive instrument that alone might reduce the value of the instrument. If it is on the inside, then you either have to dismantle the violin or find a way to go through the F holes. If you dismantle and repair, then the curve will be slightly inward (concave) so a convex bit would be a little more forgiving. But with a power tool, the slightest wobble will tear the instrument up. So I'm still suggesting you use a small hand plane, the kind that is used for guitars. The picture is a set of Ibex Violin Makers Planes.
I have also seen simple brass bodied planes about half an inch wide like the other picture. Not expensive. You could grind a convex curve into one of a set, concave in another and keep a third flat to smooth the patch. A plane wil let you go diagonally on the patch to smooth it out. But even with the miniature planes, I'd still create a clean edged shallow groove then cut the patch to that width laid in cross grain, then plane and then finish with a scraper. You can use a straight edge to guide the first plane so the edges are exact width. Since the patch grain is cross grain, it will bend into the groove so it will be pretty easy to plane flat to the surface of the area surrounding the patch. Finish leveling with a scraper, which will also give you a perfect surface you can't get with sanding.
The more I do, the less I accomplish.