Reality Check
You have to ask yourself "What's the price of principle here?"
Sounds like FM wasn't the problem - and welcomes your business. Scratch one defendant - one with deep pockets, too. Bummer.
You can't go after GCPD for their refusal to turn over any information or paperwork under the public records act because YOU didn't file a written request - a third party did, and did it over the phone. Further, there's an exception to the public records act that allows LE to deny records requests if they deem it "under investigation." Since they can wait a year before charging someone with a misdemeanor, they can deny records requests for that year. Longer if they decide it may be a felony (not that they would, but as you saw, they are pretty stupid...) Also - it's not an presently active civil or criminal court case, so no discovery requirements, and the public records act doesn't apply in court cases - discovery rules/rules of civil procedure do. No case = no discovery.
If you want to sue the officers and/or the GCPD (it has deeper pockets than individual cops, and insurance, too), you can't just run to court. You have to send a detailed notice of tort claim (yes, you'll need a lawyer) within 6 months, and then they'd have a long time before they have to deny it in writing (they will, taking their sweet time to do so). It's likely several months to a year after the incident now...time is money...what's your price of principle?
Then you file your lawsuit. Pay a lawyer up front to draft and file it, or if he takes it on a contingency, you pay if you win through his 40% (or more) cut. What's the price of principle? Oh, and it costs money for a process server to serve each defendant.
Now you can subpoena FM's footage (can't issue a subpoena unless there's an active court case). Whoops...it's gone, purged per FM's record retention policy. Or "accidentally" (they like frequent patrolling and free security). Costs money to serve them all. Oh, and you have to pay duplication costs so they can keep their original - due process and all. Has to go to a licensed/bonded duplicator - $$$. What's the price of principle?
Now you can take depositions of the cops and witnesses. Whoops - got money? Times are tough for everyone, including lawyers - who'd ordinarily front the deposition costs as they do in car crash cases, but a few minutes' detention and snotty cops doesn't smell of payoff like personal injuries, and the circumstances as you describe them, plus the possibility something unfavorable or unsympathetic to you shows up on tape or from other witnesses makes it dicey for a lawyer to throw good money at potentially nothing. What's your price of principle?
They'll file pretrial motions to dismiss. Takes money to fight them. If they win on any of these motions, the loser pays their fees (people who squawk about tort reform don't know what they're talking about - "loser pays" is in all 50 states. Idaho has negligence damages caps at somewhere around $250K, adjusted for inflation. Intentional torts can get punitive damages, but insurance doesn't cover those, so the winner doesn't end up with any more than what the loser owns, after he files bankruptcy. Getting a judgment is only half the battle - enforcing and collecting on it after appeals is hard and expensive and takes forever. At this point you'd be 3-5 years after the incident. What's the price of principle?
You'll be ordered to pretrial mediation, which in this case will likely be unsuccessful because both sides are strong-headed. Mediators cost money, and usually it's the PARTIES, not the lawyers, who pay the mediator. For three sessions minimum. All day sessions. At $125-300 per hour. You've had this going now for between 1-2 years. What's the price of principle?
Whether you file in state or federal court, your jurors will be from the Treasure Valley. Notorious cheapskates in civil cases. The jury pool is more outsiders and urban soccer moms who don't know/care about/like guns all that much, really - just because they're from Idaho doesn't make them RKBA defenders. And unless the audio or video is really bad for the cops, the mostly law-and-order, good Republican voters LIKE policemen because they keep the riff-raff out of their subdivisions, arrest drug dealers and wife beaters, and intervene to run off scary-looking or -acting people who have the audacity to be out in public...see where I'm going with this? Prayers for policemen are offered every Sunday in a whole lot of churches in the valley...churches the jury pool attends...ones with really, really big membership in the valley, if you get my drift. Do you think you will be as appealing to the white-bread urban Treasure Valley jurors as a nice policeman in the videos and in the courtroom? Do you think you can convice these jurors, who fill the local AM talk radio shows with cries for tort reform that a few minutes in handcuffs in the cold is wirth a gazillion dollars? You weren't injured, you didn't bleed...you were just embarrassed and inconvenienced a little bit, for a little while...what are you, some kind of oversensitive liberal ACLU type??? (I'm only playing devil's advocate here...easy, now...but you know someone will think it....) What's the price of Principle, to re-live the incident, get berated and insulted by the other side's lawyers, and be judged by a group of people you don't know, don't care about you at all, who by and large like policemen, who hold the key to your financial ruin in their hands, and can bankrupt you if six of them decide they don't like you...or dedide they like the cop better...or decide you are just a whiner...or who don't want to deal with AR-toting strangers in their neighborhood grocery store?
If you lose outright, you pay their fees. They will garnish your check and go after your nonexempt assets - in which case you'll have to file bankruptcy and lose most everything. What['s the price of principle?
If they offer a settlement before trial, and you don't take it, and the jury verdict for you is at or less than what they offered, if it's within a certain percentage of their offer, you're technically not the prevailing party, and will be ordered to pay their lawyers' (yes, they will have at least two) fees for the trial. Plan on them having spent 3-4 hours in preparation for every hour the trial takes. Oh, and it includes their staff's time, and all the other costs as well - exhibit preparation, photocopying, jury consultants, phone calls, faxes, research...you get the point. Got that much saved up just in case? What's the price of principle?
Yes, Garden City cops, without exception, are d*cks. It's been common knowledge for decades that they're not just d*cks, but corrupt d*cks. Apparently the people of Garden City like it that way, because they keep voting for the same people and party every election - and don't care if others don't like it. The way they see it, nobody is being forced to come to Garden City, and anyone who doesn't like it can stay away or move somewhere else. (Bet you've NEVER heard anyone express a "love it or leave it" mentality on this board before! HA!)
Bottom line: all the chest thumping about suing and 1983 suits is just that. Yes, the cops were wrong to detain you, handcuff you, and talk mean to you. No, you hadn't committed a crime and they should've left you alone. Understandably, you want to teach the d*ckhead cops you dealt with a lesson. You want to force things to change so nobody else wants to go through what you did. Noble principles all.
But what's the price of those principles? Do you want to dedicate your life to travelling to Idaho for the next few years to push this? Go ahead and roll the dice if you think it's worth the cost - I wish you well and hope you are successful.
There are other ways to get the word out and put some heat on them, ways that don't cost you a dime. Like this forum. Good on you for using it. Contact a news station and see if they're interested in telling your story - guns and gun control are hot topics right now. If they do, how do they handle the story? How does the public react to your story? If the news folks also interview the GCPD, or any other LEO, or the ordinary man on the street, how do they respond? Those would be free ways to measure how successful you might be in a lawsuit.
Best wishes to you either way, though - you do have my sympathy over this event.