I read this three time to try to figure out if I'm missing something, because no one could possibly be as dense and harmful as you are being.

You owe them the biggest apology in the world along with actual accountability (so understanding the gravity of what you've done and outlining a plan to never engage in similar behavior again) and, personally, even with that apology I wouldn't let you back into my life since you've clearly demonstrated that you are an unsafe person to be around, lack common sense, and don't understand boundaries. I really hope you're trolling at this point. If not you need serious help.

First off, it is pretty clear that Sarah did some awful things to these people early on that you are dismissing. It is not your place to say whether it is unfair of them to set boundaries due to "mistakes she made when she was much younger that she is extremely sorry for." People who are sorry don't continue to harass and abuse the people they have hurt, so the fact that you think Sarah is sorry shows a deep lack of interpersonal skills/ awareness of what is going on. Plus you are dismissing the fact that they have decided for them that they don't want Sarah around. You are making it clear you have no respect for them based on that alone and that you lack basic understanding. You say they exclude her because they have money, but it sounds more like they exclude her because she did things to them in the past. The fact that you can't seem to grasp that and dismiss whatever she did is a serious problem. They aren't unreasonable people (they tolerated you even though they knew you were taking Sarah's side early on) and their own parents think they are justified in excluding her. How dare you assume that they are in the wrong and then refuse to accept their boundaries?

Sarah tricked you into giving her a phone number she was never supposed to have and then used you and that number to abuse them at a level where they sought legal help. Think about that for a minute. That should have been your bright shining red flag that Sarah wasn't to be trusted, was willing to use you, and is highly problematic. You dismiss actual threats saying she didn't mean it?! Why are you so sure she didn't mean it? She is someone who lied to a relative to get their number and is flagrantly disrespecting their boundaries in abusive ways. They had enough to go to the police saved in their phone. All the money in the world doesn't change the fact that she made threats that were concerning enough for law enforcement to take seriously. As for this:

Amanda and Ben escalated the whole situation by filing a police report against Sarah which I think is extremely stupid because they know Sarah is harmless.

They don't know she is harmless. In fact, the very fact that she showed up at their house later (and drove around trying to figure out where they live) proves they were right in thinking she isn't harmless. She made threats against them. She deliberately obtained their number knowing they didn't want her to have it. These aren't the actions of a "harmless" person. Sarah (and you) created the situation and escalated it. The police were the best response to this situation they could have gone with.

The fact that you then told her enough to know where they live, knowing that she made threats against them, is despicable. The fact that you are considering showing up at their house anyway makes it even worse. You are enabling someone else in abusing them. You are now contemplating completely violating their boundaries further and going where you are not wanting.

You have done so many things wrong here and the fact that you are calling them childish is deeply concerning. I think you should be done with these people, they don't deserve to have to be around you. Your mistakes aren't silly, they are deeply harmful and willfully ignorant.

If you really are this obtuse, they are better off without you.