I'm going to skip on the questions answered by thorne(That i fully agree with).
@Thorne, An ideal world is purely based on personal perspectives and opinion.

-Is she asking for it, or is it a valid political statement?

That doesn't deserve an answer, sorry.

-They say most rape is done by people known to the victim. How does that fit in?

It has a lot to do with love,hate,envy,power.... you can't feel those things(or lack thereof) with strangers, in those cases it's purely about dominance/lack of self-confidence(which is less gratifying).

-Is it true that sex plays a secondary or no role in this?

Depends on the situation... in prison the dominance part just fuels the sexual pleasure, whilst in the friend/sibling cases it's a cherry on the top.

-It is said that the social barrier to rape is much lower than the social barrier to murder. Is this true? And if so, why??

In a lot of cases rape is a much worse thing to do to people than murdering them, sentencing someone to a life of humiliation,paranoia,self-loathing,Sexual objectification.... etc.
Some people look on rape victims as disgusting for being weak enough to get raped, asking for it, or just the plain old sexsim... So what.. they were just teenage boys playing around, didn't know their own strength.... and the hormones at that age!
People might lie to themselves that they don't feel that way, but in their hearths A LOT of people feel the way as stated before. People might talk the talk, but they won't walk the walk when it comes to defending rape victims.

I'd advise you to look up psychology behind rape for more info if you're really interested.