IIRC, getting medication overdose to work is a coin flip depending on the patient. I think phenobarbital works pretty consistently, that's why it's not really prescribed for its non-lethal uses as often anymore, but they still throw in a bunch of other drugs in the euthanasia cocktail to increase the probability of a successful procedure.

It's always better to end with a bang. Myth busters confirmed it.

I imagine the morphine and diazepam will make the patient calm sleepy and maybe even euphoric?