Non-lawyer here, just an old retired copper, but it would appear to me that:
1. a man was engaged in lawful activity in a public place
2. that man was physically arrested, detained for several hours, and charged with a criminal offense for which no probable cause existed
3. that man was subjected to a public trial, with associated legal expenses to defend against the criminal charge.
It would appear to me that all the elements are present for bringing a civil action against the government entities and officials involved. An arrest and detention without a warrant or showing of probable cause is a civil rights violation. Depriving anyone of their personal property without due process of law is a civil rights violation. Knowingly subjecting a person to criminal prosecution without probable cause is a civil rights violation.
As a bare minimum, this man is due a public apology from the officers and prosecutors involved (which he will never get), as well as full reimbursement of all costs for defense (which he will probably get, but it will come from taxpayers' funds, not the pockets of those who acted outside the constitution and laws).
I imagine that by the time this man has consulted legal counsel he will bring such an action, more than likely including mental anguish, psychological trauma, etc., and seek punitive and exemplary damages (also to be paid with taxpayers' dollars).
Was it advisable to be carrying a firearm in such proximity to a presidential campaign event? Probably not.
Was it lawful for this man to carry a firearm at the scene of the alleged offense? Absolutely.
To those of you following this story, please continue to post any details of subsequent actions. This could be very interesting for months to come.