Do me a favor and run your little plan by a mental health professional, a social worker, a paramedic, a 14-year-old babysitter… The moment you left those guns in the hands of my emotionally distraught neighbor for even a second after he asked for help, you lost this particular argument.
There are decent arguments for a UBC; you’re not making them. You should be focusing on what few public safety benefits would actually arise. You should be drawing attention away from the myriad administrative clusterfuck issues where UBCs fail, and toward those few scenarios where UBCs would actually benefit public safety. You should be conceding that “Universal” is completely impractical. You should be pointing out that none of the proposed "U"BC plans have ever actually been truly “U”, and that all of them have included broad exceptions in a (piss-poor) attempt to avoid many of the problems I’ve described.