Well, Shirou presumably knows she's a magus, so what does she have left to lose?
sakura's personality would lead her not to take rider back, she still, I can't say pities shinji but it's complex and to answer how to take rider back later it's simple, archer, rb, rin.
What?
Why would they need Rule Breaker for that? Plus, that would give Rider to Rin, not to Sakura.
If Rider is a danger to Shirou, all she has to do is ask Sakura, and she'll destroy the book.
And, yes, she would pity Shinji, but she wouldn't be willing to let him hurt people.