What you've read is correct. Rules may vary based on the particular airline's Contract of Carriage, but in general:
Yes. If only one of you shows up for A->B that shouldn't be a problem.
Probably. Once the other person doesn't show up for A->B their return ticket will be cancelled, however, even if booked together the two tickets should have different ticket numbers. Hence, I would expect that the ticket of the person who did fly A->B will still be honored. However, I would strongly encourage you to contact the airline to confirm.
Probably not. Unless you or the other person has status with the airline on which you are flying, in which case if you call the elite help desk, get a particularly nice agent, and really sell your reasoning for dropping A->B they may have mercy on you. However, most likely not.