Pete,
The remaining velvet will be fine to be left where it is, if that's what you desire. The large majority of the velvet is gone meaning that the remaining bit will dehydrate and dry where it is, if it isn't already. Remember though, its organic material, so if you get it wet or damp its going to start rotting, so during your process of cleaning and degreasing the skull ensure you don't get it wet. If you have the space a good choice would be to put the horns/skull in the freezer, uncovered/unbagged for at least 3 days, it will help dry (freeze dry) the velvet even more AND it will kill any fly larvae that may have been deposited in the upper parts of the horns under the velvet, which sometimes occurs. Best of luck
Joel- BRT