The lacquer is not de-horned, only the mother. Keysor Century published a white paper on this that I will have to dig out. In a cross-sectional view of the disc, the "horns" look like waves flowing out of the groove and they can actually curve over to cover part the adjacent groove.
You can separate the nickel master (father) from a lacquer with severe horns because the lacquer trapped under the nickel horn ("reverse horn"??) will break away as it is trapped under the nickel ridge that has formed over it. Once the silver is dissolved from the face of the master it is then rinsed in an acetone solution to dissolve any remaining lacquer so the master will in fact be a 100% negative replica of the lacquer. Making a mother of that is not so easy - unless the horns are not too severe so separation should easy. Press with it and try removing the record from the press when the vinyl has flowed under a ridge to replicate the horn. Now try removing the record from the press without damaging the groove.
Horns are caused by too high stylus heating, high level and ofcourse the quality of the burnishing facets of the cutting stylus and other factors plus, as usual, a combination of all the above.
De-horning is usually done by polishing with jeweller's rouge and in extreme cases using an old darning needle with a steady hand under the microscope to de-click by physically removing severe horns. As mentioned, they do actually contain audio so there is a compromise.
A stamper made from a de-horned (polished) mother is a delight to press with - the vinyl flows over the grooves beautifully avoiding non-fill and separation of record from stamper is easy and trouble-free. Another trick for separation, particularly with manual presses is to chrome plate the bottom stamper so that the record will come out on the top mould 98% of the time. This is a very quick flash plating of chrome as too much will give you a noisy stamper.
I've never heard of DMM copper mothers being de-horned as, according to the DMM cutting facility we used, the DMM stylus is not heated and so there are theoretically no horns to begin with.
