Can bereavement occur during the long goodbye of dementia?

Yes. Dementia is very hard on the whole family. The patient initially feels very frustrated, but then later is not aware of the changes. However, loved ones see the same person, who will often need a lot of attention but not even recognize them. This can lead to a complicated bereavement. It is hard for others to understand that you are mourning someone still physically there, but it is common.
Definetely. Progressive dementia is sometimes called 'an ongoing funeral".