Alzheimer's and Multiple Myeloma

My Mom is in late stage Alzheimer's and we found out about a week ago that she has advanced Multiple Myeloma, bone marrow cancer. She sleeps a lot, doesn't eat/drink very well and is bedbound, I have two questions, her hospice doc wants to start her no Seroquel for agitation but I have read it is not recommended for elderly dementia patients, what is your opinion on this? Also what is the life expectancy with Alzheimer's and Multiple Myeloma? She receives no treatment for the MM, she has a health care proxy stating comfort care only. Any info would be great. Thank you