This would be incredibly hard to enforce and police don't need another trivial thing to look out for. There already exists a "dark figure of crime," which is the amount of offenses known to the police and the actual amount of crimes that are committed. Tools like self-report data and the National Crime Victimization Survey help us understand crime and piece together how much actually occurs. However, a lot of serious crimes are already reported. Data shows that nearly all homicides become OKP, assault is usually reported, rape can be iffy, and a large amount of property crime, so long as it involves something valuable, is reported. This doesn't mean that everything gets solved, but the police are aware. However, this doesn't account for the "dark figure." I guess what I'm trying to say is that if a victim feels the need to report something to the police, so long as they are not dead, they will. There doesn't need to be any kind of coercion on the part of law enforcement to require witnesses to come forward.
No, personal safety and forced incrimination of another. I think the government is expected to prove that stuff, not require others to do it for them.
