What a very sad story. I agree with so much said on here - specifically TrendyNannys post. Your dog is an accident waiting to happen and it’s beyond irresponsible to pretend that this situation can continue. Someone - family, friend, visitor - will be bitten and you will be likely be sued for damages.
First reading I thought that your dog was 9 months. Then you mentioned annual Vet’s checks which made me think that it was the behaviour that started 9 months or so ago. I am a real ‘dog person’ and we have always had dogs - many of them rescued and with behavioural problems. It would take such a lot for me to give up on a dog but I could not and would not live in fear in my home, anxiously waiting to assess the current mood of the dogs I cared for. The suggestion by a ‘professional’ that this dog can be rehomed is unbelievable! Imagine the danger and disappointment you would be visiting on another innocent family.
There is something very wrong with your dog. Your DH must accept that. No one would have a dog in their lives who behaved like this. It could have, as has been suggested, some kind of Cocker rage. I have two friends whose very red Cocker spaniels had to be PTS due to their suddenly changing their characters and becoming so aggressive and unpredictable. One friend was bitten twice and hospitalized once for injuries before she gave in and had the dog euthanized. Poor breeding is often blamed for the dogs who display such anger and unpredictability, regardless of training and input from experienced owners.
Your dog could be unwell - possibly with some neurological problem - which would change her character completely. Again, difficult to deal with surgically and possibly incurable.
Your dog has no doubt she is number one in the family and pecking order - she has bonded with your DH but even struggles to let him give the orders sometimes. As far as you and your family and friends are concerned, forget it - she’s in charge and keeps on reminding you of that with her aggression. Strangely she doesn’t do that at the groomers or Vet or outside the home. Maybe she doesn’t need to display her bad behaviour there because she’s off duty - and not guarding her own domain!
She clearly has no respect for you and very little fear that she will fall out of favour. If she came to you as a puppy, unless she was badly bred or now has a medical problem, it’s likely that she has been really indulged and has got more and more fixed in her threatening ways.
If you could be sure that you DH would agree to her being in a kennel and run in your garden, except for walks and playtimes outside the house....a kennel with all mod cons etc., but otherwise locked safely in her run so that she couldn’t ever pose any threat in your home, then there might be a chance of keeping her without all the future anxiety and the sadness of having her put to sleep. But, that’s down to your DH really and how serious he considers the threat that she poses to everyone else.
From what you say, up to now, I’m not sure he’s as worried as he needs to be!
Otherwise, I think you’ll have to bite the bullet and give up hope that you can change the dog. I certainly couldn’t live with the worry every day that I had passed such a dog to another family to struggle with, or dismiss the very real risks to them and I couldn’t live with the anxiety of someone being bitten in my home and all the attendant grief that this would involve. Animals should give you so much joy, if you’re doing your best for them. If you’ve run out of options, you just need to be the human and make the hard decisions when it becomes necessary.