The random mood swings did make me think of epilepsy, but I suppose that has been checked for?
She sounds a very mixed up dog and though it is challenging in these situations to know what to do you sometimes have to be practical.
First of all, a dog is a companion not someone you should be wary of, or have to keep people away from.
Second, you have your other dog to think of. Izzy's behaviour will have a negative impact on your pup, especially if aggression is directed at the pup. You could end up with a second dog with issues.
Thirdly, as someone said when a dog is this bad mentally we have to contemplate if we are being fair to keep trying, or whether it would be kinder to put them at peace. It sounds like you have tried incredibly hard and done all you could for Izzy, but if even drugs aren't bringing her peace then you really need to think about her quality of life. Mental pain can be as distressing as physical pain and a dog cannot quantify it like a person can.
Lastly, from a very pragmatic view you need to think about your own safety. She has bitten you severely. Dog bites can lead to infection and there is no knowing if one time she may bite badly enough to leave other damage. My nan was bitten by a cat and I know had nasty that wound became, it was very frightening. You also have to think about other people. What if Izzy escaped? Or got away from you? Or a child came to the door and she slipped past you and bit them? If she bites another person not only are you liable financially, but you have to consider that you put them at risk by owning a dog you knew could be dangerous. In this country (UK) this has become a real cause for concern and a person who knowingly keeps a dog that has bitten a person can be sent to prison if that dog attacks someone else.
I'm sorry because that sounds so harsh, and I don't want it to sound that way. I feel for you deeply, I have friends with dogs that have bitten and they have been able to work past that, but they always have to be on their guard. My dearest doggy friend recently wondered if she would have to put her flat coat down because he was showing aggression to teenagers. She has a flat rule that if her dog bites a person then that is it. As it happened we were able to work out the root cause before he reached that level.
At the end of the day I am judging by what you have written and it does concern me that Izzy bites so hard without any provocation at times.
I hope you can find a solution, my thoughts are with you.
