Someone pointed out to me that the bracelet's double function actually was set up, since Missy evidently transitioned (using it) between the Netherwsphere and the real world. Which I have to admit makes sense. (If you think about it, other than Missy, we do not see another living being in the Nethersphere - the Doctor and Clara are both in storage rooms for the physical bodies, but not in the Netherwsphere. Missy, otoh, is there in two or three episodes when she welcomes dead people there.

...which still doesn't account for the sudden "one trip one person only rule", of course.