I agree with everything except for the part about calling the cops
Definitely call the cops and get them to appear at the event ... we should do everything we can to get authority and law enforcement of any kind at these events and to be forced to appear with these far right extremists.
Think of it
If the cops show up, they have to act and they have to do something.
If they show preference to the extremists, the cops are showing everyone where they stand
If they show any kind of moral decency, they'll do something about the extremists and dissipate the situation
Whatever happens, the public gets to see what the cops will do at these public events
Always call the cops and encourage them to appear these events ... even if you know they won't help.