” The government wouldn’t have been preventing assembly, per se, it would have been closing one venue, at most...”
Closing one venue in order to accomplish what? Oh yeah, to prevent an assembly.
“... and by enlisting the proprietor, there is no violation, period. There wouldn’t be “government action.”
The proprietor wouldn’t cooperate, so necessarily, in order to stop it, as many posters around here seem to think the cops should have done, would have required government action WITHOUT the proprietor’s consent.
Is your claim that closing off a public accommodation to a group of people is a first amendment violation? You didn't respond to the Mall of America v. BLM example, which prevents an assembly of BLM at Mall of America, and only at Mall of America. BLM can spout off someplace else.
-- ... in order to stop it ... would have required government action WITHOUT the proprietor's consent. --
My contention (which you claim is a first amendment violation) is that this would have been legal before the violence started. The government claims to have probable cause of gang criminal conspiracy.
Even without probable cause for criminal conspiracy, the police could have lanced this boil before it erupted. See Peaktastic (franchisor) Motion to Designate Unknown Criminal Actors and law Enforcement as Responsible Third Parties, in the suit against Peaktastic by Don Carlos.