Sabine is so prevalent because bombs, particularly with Ion, are one of the few good tools against /x7 Defenders.
1) Perhaps, but they're even better against pretty much any other imperial fighter that used to be popular. Which leaves imperials with few (or no) good alternatives.
2) Ion is good vs defenders because it leaves them unable to get at least one of their tokens for at least one turn. That said, pure bomber lists don't care about tokens anyway. They rely on bypassing green dice entirely.