With regards to point (b) though; as I've written elsewhere, it is absolutely vital -- in my opinion -- that a person's loved ones feel in control of the body of the deceased. Denying them that control may potentially, in some circumstances, result in significant trauma (specifically in the case of those who are experiencing denial and have yet to work through it).
This is why I completely reject the idea that "maybe where we can we have to be encouraged to be hard and fast about it". Under a system of presumed consent, organ banks would see a massive increase in donations above and beyond current levels anyway. Other countries have demonstrated that very few people / families (percentage-wise) refuse to allow organs from a dead relative to be used. Most people see it as a way to salvage something from the death... the idea that hopefully someone else will get a new lease of life. But for those who don't feel that way; for one reason or another; it is completely inappropriate for a government or organisation to assert ownership of the body and usurp their control. To do so could cause lasting damage in my view.