Yeah, in the UK they reduced the amount you can buy at once too, so drugs like paracetamol and ibuprofen only come in 16 tablet packs and you can only buy 2 packs at a shop., It's helped lower suicides quite well.
People here have a real aversion to taking tablets a lot of the time. I have to convince people to take regular paracetamol (which is non addictive and doesn't have side effects so long as you don't take more than the recommended amount) after they have had surgery semi-regularly.
So people with end stage cancer do often deteriorate like that, the morphine is unlikely to have caused her death but it would have definitely made her more comfortable. You did the right thing for your grandma. You didn't speed up her death, you cared for her through a difficult time. Thank you for being there for them.
If this is playing on your mind get in touch with the hospice/ palliative care team who were involved and talk it through with them. They will be happy to help and won't mind you calling at all, even if it is a long time since it happened.