Do you have to attend the “mediation”? I do understand the principles behind it, but I don’t think it’s for everyone, and I would certainly not feel disposed to “mediate with” someone who’d called me “a crippled c*nt” - or anything remotely similar.
Mediation suggests there is common ground to be reached, or perhaps even that there has been error on both sides. If it was completely unprovoked, and I didn’t feel it was part of some feud for which I’d been at least a tiny bit to blame, I can’t really imagine what common ground I’d have with such a person.
I’m sure we’ve had this discussion before, but I think, in your place, I’d have been tempted to go straight to the police by now, and bypass work’s attempts to “mediate” altogether.
What the person is doing is a criminal offence of causing harassment, alarm or distress, and you have also mentioned or hinted at assaults. That isn’t a workplace spat, it’s a crime or pattern of crimes, and there’s provision for harsher sentencing if it’s proved it was motivated at least partly by disability - just the same way there is recognition of racially aggravated crimes.
Work aren’t above the law: they can’t stop you going to police, and force you to accept their “mediation” instead, if what’s happened to you is a crime.