I think there's some false premises you're working on, and some sophistic ones as well, and I'll do my best to address them, understanding that, as you've pointed out, this is a hot button topic, so most people won't want to engage in any way.

1) You're correct, there's far more categories for humans than simply "Minor" and "Adult". For the purpose of this specific conversation and the laws surrounding it, and to avoid constantly having to shift, mitigate, forgive, condemn various actions based on the individual or the minute changes in age and circumstance, we create these categories and stick to them - The, somewhat arbitrary, age that we agree to be Too Young and the age we agree to be Old Enough. It isn't based on some strict definition, and it would be almost impossible for it to be so. Instead it's a handshake where society says: "We'll never know the perfect age for each individual to start doing various adult things, so we'll all just agree that it be age XX and leave it at that".

This isn't just satisfactory, it's necessary. The same as we don't modify the rule of law for every individual, but rather allow the justice system to judge them based on their individual circumstances, we need to do the same for these laws. We don't make it illegal to steal unless your Dad is dying of cancer and you need money for his treatment... But when you get to court, we expect a judge to be more lenient on someone in those circumstances. By the same token, we all know that Minor and Adult are chosen at a good guess at best for the purposes to determining an age limit on certain activities, but we expect a judge to take into account the circumstances around it.

2) True, but, again, for legal purposes, you could continue this divide forever. Pre-pubescent kids, toddlers, infants, kids currently going through puberty, people who are the right age to start puberty but haven't, people who started puberty young, people who haven't gone through puberty but are very intelligent and mature... The list could stretch forever. While it's not perfect, we accept that the legal treatment of these people needs to be general and allow the court to make their own judgements.

3) Obviously this changes from person to person, but it's the same as point 1 - We don't all believe that people who are 17 years, 364 days, 23 hours, 59 mins and 59 seconds old are mindless, drooling children who, in 1 second time, will become intelligent adults. Instead we agree that there's no single point where that's universally true, but there needs to be an agreed point for us to treat the law and the people equally. Otherwise, when people started turning 12, we'd be having yearly tests of their maturity by phycologists and doctors to see if they are adults yet, and then there would be primers on how to pass those tests going around online... It isn't viable for us to expect the world to treat every individual as an individual in this broad a sense.

4) Very few people think that, but we agree that children are ill equipped to understand these things. They don't have the experience or the understanding of the world, are still very much driven by basic wants and desires, don't have a good grasp of the future or what the world expects of them, so on, so forth. There's a reason we have to teach kids not to play with fire, or not to go out and party all night before an exam. If they were capable of understanding the harm their actions were going to bring them, it wouldn't be necessary to protect them from anything... But that simply isn't the case.

5) See above. It's not impossible any one child might know what they want, but we understand that children in general don't know what they want. They are short sighted and impulse driven. It's not just a stereotype. You might say you know a child that is mature, intelligent and knows what they want in life... But the fact will remain that, if you have 1,000 kids in a room and threw a stone, the odds are you'd hit one who doesn't. As a result, we need to create our laws and rules around the understanding of what is true typically and let the individual issues become a matter for a court or jury to determine.

6) See above. Same thing, different window dressing.

7) To give you the benefit of the doubt here, I'll assume we're talking about someone who has physically matured and is attempting to look older - make up, clothes, environment, etc. Yes, accidents can happen. If you're talking about before a person matures, I would say that's unnatural, yes, and I think most people would agree. That being said, because we don't have much choice but to pick a time when a person is considered an "Adult", we have to treat them in this way. If your point is: "Why do we punish them if we agree it can happen?" - Because we need to have this agreed age in order to create reliable laws, we have to rely, then, on the legal system to judge if the person's actions were reasonable and forgivable.

8) See above. We know they can, we accept that they can, we just acknowledge that, as a rule, the majority of them aren't mature enough to understand or control what they feel. We also agree that a mature, intelligent adult can manipulate and control these children, meaning that they can easily take someone who isn't ready, convince them they are and be heedless of the results. Is it always true? No. Is it true often enough that we have to worry about it and make laws to prevent it?... Yeah, sadly.

9) That's the same point as the two above, just a different coat of paint.

10) I'm not sure what you're saying. We agree that children can sexually assault each other, we just treat them differently under the law, as mentioned above.

So, in essence, what you're arguing is that there's more nuance to life than we recognise in the law, and that's always true. It's why we don't simply have a computer that we input your crime and receive your punishment. We use a court system so that the law can be general to treat everyone equally, and then the outcome can be moderated by a human who understands that there's circumstances which differ from person to person. If your argument is that pedophiles don't get treated with much nuance - Hard to say you're wrong, but I totally understand why. Children are innocent, easily led, and things like this can easily ruin their growth and alter their whole lives going forward. The same way we naturally react with disgust at the murder of a child, we naturally act with disgust at the harm of a child in this way.

What it might help to argue is that there needs to be better methods of treating people who are pedophiles and agreeing on helping those who seek treatment without judgement. It's just one of those fucked up things nobody asks to happen to them, and if someone is trying to get help, good. That's what we want, right? Not for them to hide it until they act on it, but to try and fix it. It's till gross... If I found out a friend was like that, I'd still be repulsed... But that's half the issue.