Jan, I think the explanation for the double BAHAs of Mo's brother is that with AN, you have sensory-neural, or inner ear hearing loss - no functioning hearing organ or nerve. The BAHA then serves to transmit sound through bone to the other side.
But many people experience conductive hearing loss, a problem in the middle ear. Their inner ear is working fine, but the sound just doesn't get to it, and an ordinary hearing aid won't help. In that case, a BAHA can transmit the sound through bone to bypass the middle ear, so it gets to the inner ear on the same side. if someone has conductive hearing loss on both sides, then stereo BAHAs would make sense for them.
Elda's is the first case I have heard about of a BAHA installation going awry. I think it is pretty rare, but it does sound like the doctor doing the procedure should make sure that the bone in the area has healed well before proceeding. Elda, you have my best wishes for a speedy recovery from all this.
I suspect that the installation was not done by the same doctor who did the AN surgery, so they may not have known as much about how the retrosigmoid surgery might have affected the bone. Hopefully the situation can be straightened out in Elda's case, and can be avoided for others. I hope it does not put people off too much, because there have been a lot of successes with the BAHA and it has been a big help to many users.
Steve