If used properly a plimsoll will leave marks on a bottom for some time, not as long as a cane though. At school when changing for games I saw plimsoll marks on bottoms where you could see the pattern of the rubber tread!
Yes would of been good to see her get more of the slipper, it would also have been good It to see the tables turned on this mature slut and see her forced to bend over, her skirt lifted up and given a good spanking over over panties with the slipper, then her panties pulled down to admire there work for a moment before giving her another hard spanking with the slipper to this mature fucking mistress's bare ass, but harder and more than than she give the young slut, do you not think.