I'm not gonna lie I'm at the point where I see a wall and I'm like "whew". I know my posts are walls too, it's become quite intense.

I don't think we disagree on a great deal. I'm gonna skim through your post (I have read it in full) and respond to a couple of bits in the hope our posts aren't so huge and intimidating!

The way law works in your country, it seems strange. We vote people in, they have the power to make laws. If a law changes, if something that wasn't illegal becomes illegal, you can't use it your defence "other people got away with it before it was illegal".

A party should be able, in theory, to come into power on the promise of changing a certain law. They should be able to live up to this promise. This is how you make sure law is fair. If it's not, it can be changed at the ballot.

Negotiators in hostage situations are not armed
Later you say this...

I'm suggesting that unarmed police be the first on the scene with an armed entourage. I'm not suggesting that only unarmed cops should be sent anywhere.
Ok, fine, I'm cool with this. I was going to make the point that hostage negotiators have armed support, but I don't need to.

This seems hyperbolic. You're serious, though, yeah?
I was making this comment on the presumption that an unarmed first respondent was alone. The first time it happens, immediately we're asking questions of how different it would have been had they been armed. But if we';re talking about armed support, it's different. The criminal is still under the gun, and if he is going to shoot, it's highly unlikely he goes for the only unarmed person.

I have no idea what you're trying to say, here. I know nothing of the crime problems nor the corruption if any in Brazil or Rio.
I was making the point that USA as a whole is comparable to half a city. Considering USA is an armed nation, politically divisive, with a history of school shootings and domestic terrorism, 700 people shot dead by cops in a year isn't huge.

Of course we want that number to be lower, but we're being somewhat naive if we think that figure can be zero. How low can it realistically be? For sure lower than 700 because there are clearly people getting shot that didn't need to be, for example our drunk sleepy guy, but can we really expect a much lower number? There's a lot of fuck ups in USA.

Seriously. Anyone who is denying that there is systemic racism in the US is simply not even looking to see whether or not the history bears that out.
Why aren't people like Biden doing more to fix the systemic racism? Nobody expected Trump to, but what's the point of Biden if he can't fix this? What the fuck was Obama doing? How can it ever change in USA if Presidents can fix it? At the very least they can speak out about it. I don't ever remember hearing Obama telling the world how institutionally racist the American system is. If he can't change the law himself, he must put pressure on elected officials in the Senate or wherever laws get repealed.

How can it ever change if even a black President doesn't even try to fix this?

They just don't like it when a "bad apple" gets their cam exposed and that's the imagery in the public eye of who police are and what they do.
This is the only way to win public confidence. Nobody expects the police to be able to get their recruitment and training absolutely 100% tip top. People expect accountability when they do get it wrong, and that means prosecuting the bad apples, firing the incompetent, and bosses resigning when they make bad decisions. If the police know their bodycam footage is available to the public on demand, they will be much more likely to act with restraint and professionalism. It would be a huge step in the right direction.

This is already turning into a wall.

I'll just nod my head along until this...

Exactly the false assumptions that need to be explored for what they are.
I'm not so sure it is a false assumption.

Humans have lived in densely populated cities for thousands of years. Human morality hasn't changed in that time. The human genome has been relatively stable for tens of thousands of years - maybe hundreds of thousands. So the genetic propensity for crime and immoral behavior hasn't changed. The environmental pressures by-and-large haven't changed.
The cities of the past (200 years plus) are incomparable to the cities of today. Our societies are completely different. The most obvious difference is money. Also drugs. Mental health. Wealth inequality. Guns. These are important factors when we talk about crime.

I know these things aren't completely new to our times, but with larger populations it becomes a larger problem. Of course there is much more crime now than in the past. Without law enforcement, you have vigilantism. Of course civilised societies have evolved to have a police force. I don't think it's ridiculous to assume the alternatives would be more unpleasant.

So when we see the unequal treatment in law - when we hear the people who penned the laws admit that they designed the law to persecute Americans - that's a problem. A serious problem. A threat to democracy problem.
Well clearly you don't have democracy. You can't change anything by voting.

I'm trying to understand how there are these vastly different world-views about what the police are and what we as a society get for having them.
I'm obviously heavily skewed by what I observe in the UK. The police here, they can be dickheads of course, but if you don't piss them off, you're not going to have a problem. The only people who really have to worry about the police are criminals. That makes society safer. So we get value from them. Is that value optimal? Highly doubtful. But I understand where the value comes.