There is no (good) reason to limit yourself to points, you can see if your buffer intersects with the countries directly:

    public SimpleFeatureCollection lookup(Geometry g, SpatialIndexFeatureCollection countries) {
        GeometryFactory gf = new GeometryFactory();
        
        Filter f = ff.contains(ff.property("the_geom"), ff.literal(g));
        ret = countries.subCollection(f);
        
        
        return ret;
    }