I think, even if they did end up stabbing her, that can't have been what they intended to do. Minatsuki's card does say she can control her blood, so giving her blood to work with would be really dangerous. While her pupil's condition does match hanging, you're right that the bruises are really light, but we can probably extrapolate out that if a hanging would cause it, damage to the neck would, too. Could the bruise be from someone kicking or punching?

I think we should also consider trying to figure out which of our suspects could have made a climb like that, since climbing seems to have been involved. It would have to be someone who was absolutely certain they could manage it, I'd think.