First of all, so sorry that you are on this road.
My husband had unrelenting nausea near the end. The doctors blamed this on the tumor cells invading the nausea centers of the brain. He had cancer cells in the CSF fluid.
Anyway, hospice told me that Haldol, which is an anti-psychotic drug that they gave me, also works for nausea. So you may want to try that. My husband was on so much morphine, Haldol, and Ativan towards the end that I can't say what was doing what.
In general, morphine just makes them feel better so it can't hurt. I was thanking God for morphine near the end.
God bless you as it is so tough what you are doing, Sally