I am sorry your husband has plasmacytoma! It is very bad for him to have both fatal diseases. You are right, at this point, quality of life is important. No more aggressive treatments unless you want him to have.
Sometimes antidepressant or antipsychotic drugs can help his delusions. Also, don't make him watch the news or read the bad news. Sometimes he gets confused with the news and feels that it is real in his local area.
My FIL has stage 6/7 Alzheimer's and he was diagnosed with it in 2006 when he was 86. In 2007 when he watched CNN and saw this Virginia Tech killing, he thought the killer was nearby in Chicago and was afraid that he was in danger and all that. The caregivers had to distract him. He was still watching TV but they tuned it in with Animal Planet or adult cartoons (the Simpsons or Family Guy.)
How you manage what he receives everyday from the media does make a difference.
It also depends on his life style - if he was a police or a news reporter and that kind of profession... These elders don't forget their professions whatsoever.
My FIL was a professor in Chicago and a doctor in his home country. He still thinks he wants his sons to work with him in the nursing home. He always thinks work is more important than family. He is married to work. (Actually he has had this delusion for 30 yeas.
) For exmaple, he had done experiments on cats in most of his professor's career so he would think the overnight caregiver at home was to do experiment on him overnight. He would think the hospital tubings are for experiments and he does not want that. He imagines that he is working in the home and that the blood test for a check up is for "work/experiment"!
He would not have delusions out of the blue like a real mentally ill patient.
Once you find out the clues to deal with, he would have better quality of life in peace.