The way I see it, the man was a police officer. He was trained, sworn in, and worked that job (apparently) in good faith. Obviously he got the job under false pretenses and should be immediately fired, benefits pulled, etc, and appropriate charges filed against him.
However, I would argue in the inevitable appeals that because he performed his duties in good faith as a sworn officer of the law and followed police procedures, the people he arrested are not entitled to automatic release. (although obviously there's going to be investigation on those arrests to make sure there was no improper actions taken)
That said, I have no idea what the law or court precedents say about a situation like this, because I don't think I've heard of a case like this happening before.
With all due respect, I don't think you're grasping the significance of this. You're hanging on to "he was sworn in, he was a good cop, etc etc."
None of that matters. Just like if it were ever to be proven that Obama was not eligible to be president, it doesn't matter if he was the best president in history, everything he did while president would be immediately null and void. All bills he signed, all laws, everything. It would be like he was never president.
It's the same thing with this cop. I don't give a damn if he was sworn in and made USA cop of the year. He was not eligible to be a police officer. He was not eligible to make arrests. Every arrest he made should be immediately voided.
YOU CAN'T BE ARRESTED BY AN ILLEGAL ALIEN, I DON'T CARE WHAT UNIFORM HE IS IN!! You can't say "well he was a good cop so we'll just fire him." There are people in jail right now who were arrested by a cop who wasn't a real cop and wasn't eligible to be one. That is HUGE. I can promise you tons of lawsuits are going to be immediately filed.
If he was sworn in under false pretenses, then his swearing in was basically "junk." It wasn't worth the time to listen to it. At the time, nobody but he knew. But that doesn't make it "ok."
You obviously don't understand how our laws in this nation work. To be sent to prison/jail, you MUST have charges filed against you, and every law along the way must be followed to the letter, else the guy walks. Whether he did it or not, whether he was found guilty, it all starts with an arrest. If that arrest is false, that's like pulling the foundation out from under the house, the entire case falls apart immediately. That's how it should be with everyone he ever arrested. And, everyone who was ever arrested by him should be able to sue him for tons of reasons and get tons of money out of him. Can't get blood out of a turnip? Cool, but make damn sure that every penny he ever earns in the USA goes to the people that he falsely arrested and put in jail.