Lacquer is seen as protection, but this is only part of the story. The sanding process, using finer and finer grades of wet'n dry paper is to remove the surface damage, scratches, crazing and milkyness, not to achieve a clear "as new" finish. After sanding, the surface should be smooth, but not too smooth, like the colour coat on flatted paintwork. This flatted surface provides the key for the lacquer to bond to the polycarbonate. The lacquer will turn the flatted surface clear, just as it transforms the flat colour coat on paintwork.
Maybe some of the problems experienced with lacquer only lasting a short time is due to the over enthusiasm at the sanding stage, to achieve a clear finish before lacquering, rather than preparing the surface for lacquerIng.
If you can find a local body shop who will do it for you, I'd think that would be a good idea.
Mac.