Biting bichon

We have an almost 13-year-old bichon that we bought from someone we thought was a reputable breeder.  However, she had gotten greedy and bought a litter from a puppy mill and passed them off as her own.  Because of problems and complaints, the rest of the litter was destroyed but we had bonded with our puppy and decided to deal with the health/behavior problems we were told to expect.  We had her spayed and did extensive behavior training.  One trainer who has trained dogs for clients all over the world finally gave up on her and said he had never done that before.  She even lived with him for a month with little success.  She has always been aggressive and a biter.  Everyone coming into our home is warned to ignore the dog and/or she is put out of contact.  She is especially bad with chldren because they move quickly and with anyone who tries to get close to either my husband or myself.  She has never been in the same room with our 18-month-old granddaughter.  At one time or another, all the family members have been bitten, usually without provocation or serious injury.  Today she badly bit my husband for no apparent reason while sitting on his lap.  Her behavior is becoming more unpredictable lately.  Because of her age and history, I don't know what to do other than have her put to sleep.  She is well cared for and has no medical problems other than a skin condition that causes her to scratch.  I hate the thought of putting her down but we can't live with her not knowing when or whom she will bite next.  I would welcome any suggestions.  Thanks.