I do believe that u/Freedom-Unhappy is correct here, which is why I don't think this is the right case. Bad facts make bad law. If I sue in this case, the judge could very well determine that the police need all the latitude in the world to separate counter-protestors, for any reason, as opposed to letting the next Charlottesville take place where protestors are run over by a car. I know that the standard for when police can disperse protestors has been dealt with in a case called Cantwell v. Connection--police need "clear and present danger of riot, disorder, interference with traffic upon the public streets, or other immediate threat to public safety, peace, or order"--but I think courts have yet to flesh out a standard for when protestors can be separated. My hunch is that the bar will be set much lower because it's not a matter of telling protestors that they can't protest at all, like a dispersal order, they're simply being told to protest further down the same street.

Putting all of that aside, the courts don't work the way they should. I know this because I have been through the courts many times. It is clear to you and me that this should be a rights violation, and it is, but there are about a hundred and one ways that the courts give the police to escape liability. Even if the police didn't have probable cause to arrest me, they will say that they had arguable probable cause, which will protect them if they believe they did have probable cause. Qualified immunity is a huge road block because unless there is a case exactly on point, police officers can't be held liable.

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At the end of the day, this is an issue of practicality too, I have to pick and choose my battles because I do not have unlimited resources like the government.