I am going to tell you some things that make perfect sense even though it will totally conflict with all western society has been tainting your mind with.
1. Downplaying the seriousness of it -- Ummm....no. In the west we go on and on about how women MUST be totally traumatized if such a thing happens and they can never heal. Both are not only untrue, its bad policy to say so. It makes women who are not traumatized feel like something is wrong with them. It makes women who are a bit traumatized have difficulty healing.
The point of that clap trap was to make men feel supremely guilty if they did any sort of "wrong" to a woman. Of course it works on "normal" men who probably would not have harmed a fly anyway. If they touch a woman accidentally they act like they are now demon spawn. Naturally the men the idea is aimed at hardly take notice.
No, I think if the actual victim is not saying its such a big deal, you should just agree, rather than try and convince her her life is over or she needs a shrink or she is damaged goods now.
2. This idea of a lack of equal viewpoint. Poppycock. The west is biased towards women. Fact is with the overwhelming majority of alleged sexual assaults of any kind, it is horribly hard to prove there was an assault. Most times there are no witnesses and no video or audio recording. Bruises could be from anything. DNA could be present because of consensual relations.
In the west though, the woman's word is so often taken from the get go. The female accuser gets anonymity. But the accused male gets his name in print and tends to lose his job and maybe even his family before a trial has even begun. Convictions lean heavily on the word of the accuser.
Fairness in the west is being described as bias of the patriarchal society. A great example of all this is Josh Duggar. He has admitted to molesting up to five teen and pre-teen girls as they slept. Thing is, he did this as a minor himself. No one pressed charges against him. He admitted it openly and begged forgiveness. But what happened? A newspaper dug and got his name and his entire family got punished with the cancellation of the family show. Fairness? Heck no. Western law is misandrist. But I will say this: Japanese law is rather random so I am not going to say its perfect.
To your questions:
1 Do women report it or is it practically useless? -- Under reporting is the norm everywhere. Don't be fooled. Reporting is generally WORSE than useless. Her reputation and self esteem will both take a hit for people knowing and talking. Lawyers cost money. Convictions are hard to obtain because as I said before, proper evidence is generally lacking.
2. Is there a culture of shame associated with being sexually assaulted / raped? -- The shame is the same as in the West. Its not shame due to sexual assault. Its shame caused by people around the woman being aware of her sexual past. I think it is a natural sentiment BUT, that natural sentiment is enhanced in the West, especially in Anglo societies. Thus, women tend to bury it if possible.
3. Do people hide the fact rather than seek justice? Considering the traditional culture of not being open. -- I don't think its some unique Japanese or Asian culture thing at all.
4. Is the younger generation better? -- As in less likely to commit such assault? I believe so. But I think it mostly has to do with plummeting amounts of testosterone and maybe even addiction to women that are not "real", such as comics and porn.
5. Why the do people let it happen? Just stand by? Not do anything? -- Usually they don't know its happening. They don't see it.
If they do see it happening, how do they know its not consensual? Its not like the women plead for help or scream NO or anything. You would feel awful embarrassed to interrupt what turned out to be consensual. And here is an awful, awful truth but...if you do intervene you won't get any thanks. Thus, you won't even know if you saved the girl or interrupted. You will go home totally confused and you probably won't do that again.
6. I find this hard to understand coming from a place where sexually assaulting a woman would get you punched square in the jaw (or worse).--- You watch too much TV. The idea that things are so bad in Japan but so wonderful in Anglo societies is nothing but propaganda. The media goes from declaring Anglo societies to be a paradise for women one second and without missing a beat talks about how terrible all these rapes are and how rape rates are rising or what have you.
And women are not so very protected. Most of the times I have seen people get physical or nearly so concerning women, it was nothing but jealousy. Or it was a mistake.
Anyway, the media is chiefly interested in using this emotional topic to sell print. People love the topic in an emotional way. Logic hardly ever enters the picture and when it does, people get angry cause thinking logically steals their rush.
Also, logic hinders the female ability to twist situations to her advantage through lies and deception. Women don't want men thinking they enjoy sex. If that happened you might have doubts when they scream rape as a way to cover the fact they cheated on you or to protect their reputation or to harm the reputation of a guy that turned the down or dumped them.
And don't think I am totally on the male side either. I did point out violence and jealousy earlier no? And its guys that have so much trouble getting over a woman's sexual past, necessitating that she bury it as best as possible. There is stupid unreasonable garbage on both sides of this fence.