So, ULTIMATELY speaking, whose "fault" was it?

Was it really Dolores' poor bus driving and getting distracted on that day that caused the bus to crash? Or was it, like Ian Holm's lawyer character suggested, really a matter of someone putting a wrong bolt on the bus or the guard rail not being legitimately "strong" enough? Thanks.

P.S. While watching this film, was this basically SPELLED OUT or was it left somewhat "ambiguous" in the film and up to the viewers to decide? Thanks.