If grandma is at all familiar with human ashes the wood ashes switch won't work. I think that anyone who has dealt with human ashes would guess the fakes.
I've had to transport human ashes twice, and they were very heavy for the size of the box. The first time a very kind man lifted the "person" into the overhead compartment on the plane for me. He must have wondered what I had packed, but I didn't tell him. I was very fit at that time, but couldn't lift the heavy box over my head. The second time I needed to transport ashes I just left them in my car trunk, until I got to the cemetery.
I never opened the plastic and looked at the actual ashes. Gramma dishes, who did see them, posted that they are rather like ".....tiny itsy bitsy pebbles....." Wood stove ashes are like dust - at least at our house.
I think that I agree with SPuck, who said.....
I think in the end, no one should guilt or even politely suggest the grieving DIL into any action she does not want to do nor would her husband want to do. In the end it is going to be the OP's mother who has to deal with her own lack of spine in the relationship she has with her mother, no one else.
I feel very badly for OP's mother, who has this difficult parent to deal with. Perhaps her health will be better if she learns to say - no - to her mother sometimes.
My mother will probably have some major stress-induced health crisis.....
is stated by the OP. If grandma is 99, then mom is likely in her 60's, and doesn't need any more stress.
It would be difficult to change a lifetime pattern but perhaps even taking some small steps would help OP's mother.