This is my first reply too.
I feel your hurt and pain.
I hear you frustration, and your protectiveness of your son, and yourself now.
I sense your love for your friend and husband, who you miss, and your sense of loss, and of caution not to upset the but angry, insensitive, uncaring and unpredictable "black dog" which has replaced him.
Is this true?
Do you feel threatened or vulnerable?
If you do, that is a valid reason to seek help.
Confronting dogs who are upset corners them, and may make then over-react, or defensive, or aggressive, purely with instinct of personal protectiveness.
I have cared for an angry cat lady wife for 20 plus years also. She is my best friend, but wow do her claws, and her growl hurt sometimes. For me it is her withdrawal though that hurts more.
I care for two teenage kids also, both with varying health conditions. I have learned that the ugly cat/dog is not really them. It comes and goes. But the more territory they get, the more they demand, until they become dominant and controlling.
Im sure you understand.
I applaud you for being brave and courageous to not only want to stop the dog, and protect youself and your son, but to also at the same time, want to care about your husband who is still frightened and vulnerable within the dog persona, and who you still care about.
I agree with Quercus' reply ... to read about support in the forums. You are not alone!
In particular I suggest you read this article from Carmela ...
Forums / Supporting family and friends with a mental health condition (carers) / Are you supporting a depressed partner? My tips from 18 years of experience
Especially first two points, about seeking support, and about Relationship boundaries. I think this is valuable advice for any carer.
Talk with trusted friends and seek support for yourself first. Talk with Beyond Blue or Lifeline if you need to. With your GP! All these people can help support you on your journey of taming the black dog/cat that is depression.
PS: these dogs/cats can be sweet and loving and full of joy also ... they sometimes just need TLC, or space.