I think that in the world of MPD we are probably meant to conclude that it is Rouse who cheats, but that the case isn't proven, and that Miss Pym's self-interest subsequently lies in its being a bit ambiguous, because otherwise she would have to conclude that the First Cause of Murder isn't wholly Miss Hodge's decision to send Rouse to Arlinghurst, but her own wasted opportunity to prevent this Miscarriage of Justice.